Optical Absorbers

ABSTRACT

Optical absorbers and methods are disclosed. The methods comprise depositing a plurality of precursor layers comprising one or more of Cu, Ga, and In on a substrate, and heating the layers in a chalcogenizing atmosphere. The plurality of precursor layers can be one or more sets of layers comprising at least two layers, wherein each layer in each set of layers comprises one or more of Cu, Ga, and In exhibiting a single phase. The layers can be deposited using two or three targets selected from Ag and In containing less than 21% In by weight, Cu and Ga where the Cu and Ga target comprises less than 45% Ga by weight, Cu(In,Ga), wherein the Cu(In,Ga) target has an atomic ratio of Cu to (In+Ga) greater than 2 and an atomic ratio of Ga to (Ga+In) greater than 0.5, elemental In, elemental Cu, and In 2 Se 3  and In 2 S 3 .

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.61/785,482, filed on Mar. 14, 2013, which is herein incorporated byreference for all purposes. This application is related to U.S. patentapplication Ser. No. 13/595,730 filed on Aug. 27, 2012, which is hereinincorporated by reference for all purposes.

FIELD OF THE INVENTION

One or more embodiments of the present invention relate to methods ofmanufacture of optical absorbers suitable for use in solar cells.

BACKGROUND

Optical absorbers for use with solar cells are more economicallyattractive if they exhibit high efficiency and can be made from thinfilms. Absorbers based on various combinations of at least copper,indium, gallium, and selenium (CuIn_(x)Ga_(1−x)Se₂ or “CIGS”) have beenwidely studied to meet these performance goals. CIGS has a strongabsorption coefficient for visible light making it possible to usethinner absorber layers further reducing costs of assembled solar cells.

This composition is often described as having one Cu atom for every Inand/or Ga atom (i.e., Cu/(In+Ga)=1, where the atomic symbols refer hereto the number of each type of atom). However, high-efficiency absorbersare Cu-poor (Cu/(In+Ga)<1). For example, Weber et al. describe thepreparation of a solar cell using a metallic precursor layer depositedby sputtering from a Cu₈₅Ga₁₅ target and an In target to produce aselenized absorber having a Cu/(In+Ga) ratio of 0.8 (Weber, A. et al.2011 “Fast Cu(In, Ga)Se₂ formation by processing Cu—In—Ga precursors inselenium atmosphere” 37th IEEE Photovoltaic Specialists Conference,Seattle, Wash.; 19 Jun. 2011). Absorbers are typically also Ga-poor,having a Ga/(In+Ga) ratio<0.4.

However, Cu(In,Ga) metal precursor films (used to form CIGSe viachalcogenization) with these preferred atomic ratios are multi-phasicand tend to separate into discrete domains when deposited, especiallywhen exposed to processing temperatures above about 155° C. This phaseinhomogeneity can be observed in X-Ray diffraction and also in variousmicroscopy techniques, such as optical microscopy, scanning electronmicroscopy, and atomic force microscopy (the roughness tends to gohand-in-hand with the multi-phasic nature of the film). For example,Weber et al. describe that at room temperature, the metal precursorlayer contains the crystalline phases In and at least oneCu_(x)(In,Ga)_(y) phase (though not clearly assigned), and that uponheating, an In melt is formed, with a resultant decreasing In/Ga ratio.This phase separation makes it difficult to form laterally uniformcompositions, and after selenization, the resulting CIGS absorbers arealso non-uniform, reducing the achievable open-circuit voltage and fillfactor, and therefore, the overall performance (efficiency).

Attempts have been made to fabricate laterally uniform CIGS layers byphysical vapor deposition (PVD) of the metals (followed bychalcogenization) by sequentially using sputtering targets such as Inwith alloyed sputtering targets such as Cu_(0.75)—Ga_(0.25),Cu_(0.60)—Ga_(0.40), or Cu_(0.85)—Ga_(0.15) to force uniform depositionof layers having the desired composition. However, upon deposition orupon subsequent heating, the film always comes out laterallynon-uniform, as indicated by optical and electron microscopy.

U.S. Patent Application Nos. 2012/0313200 to Jackrel and 2010/0248219 toWoodruff describe that even if a uniform precursor layer is deposited,the materials may flow and migrate during processing to result in adifferent layer uniformity, roughness, homogeneity and quality andnumber of crystals within the layer. These publications describe the useof particles mixed with a carrier liquid to form an ink and the ink isused to coat a substrate to form a precursor layer.

Further performance improvement in CIGS absorbers can be achieved bygrading the bandgap across the thickness of the CIGS layer. As outlinedabove, it is challenging to control the lateral uniformity when startingfrom today's solutions for Cu(In,Ga) sputtering followed bychalcogenization. Controlling both lateral uniformity and compositionaldepth grading for CIGSe or CIGSSe, therefore, has proven even morechallenging. Other CIGS growth methods try to improve the control overboth lateral uniformity and compositional depth grading. For example,grading can be achieved by varying the In/Ga ratio through the thicknessof the CIGSe film. This is most often done by co-evaporation which hasproven to be challenging on the manufacturing floor. Similarly, thepreparation of graded absorbers can be performed using chalcogenidetargets. This, however, results in a decreased sputtering rate comparedto metal deposition, decreasing throughput, and increasing capitalexpenditure, in addition to an increased cost in sputter targetmanufacturing compared to metal targets.

SUMMARY OF THE INVENTION

Optical absorbers in solar cells and methods of forming opticalabsorbers are disclosed. An optical absorber is part of a thin filmstack in a solar cell. The absorber layer is a CIGS(Se) semiconductor,formed from a precursor film stack which is chalcogenized. The precursorfilm comprises one or more thermodynamically stable layers comprisingCu, Ga, and In, wherein at least one layer comprises a layer rich in oneor more of Cu and Ag, i.e., (Cu+Ag)/(In+Ga)>1.0, wherein the overallaggregate composition of the layers forming the precursor film is0.7<(Ag+Cu)/(In+Ga)<1.0, 0.0<Ag/(Cu+Ag)<0.3, and 0.0<Ga/(In+Ga)<0.5. Insome embodiments, the overall composition of the layers forming theprecursor film is 0.7<(Ag+Cu)/(In+Ga)<1.0, 0.05<Ag/(Cu+Ag)<0.3, and0.0<Ga/(In+Ga)<0.5. The composition of Cu, Ga, and In in each layerexhibits a single phase and the phase remains substantially constant incomposition and laterally uniform in composition when heated above 155°C. The precursor film stack can comprise from one to ten or more layerswith the precursor film stack typically ranging from 400 nm to 800 nm inthickness. Upon chalcogenization, the precursor film forms an opticalabsorber that is typically from 1.0 μm to 2.5 μm in thickness.

Methods of forming an optical absorber comprise depositing a pluralityof precursor layers comprising one or more of Cu, Ga, and In on asubstrate to form a precursor film, and heating the layers in achalcogenizing atmosphere to effect a chalcogenization reaction. Eachlayer in the plurality of precursor layers comprises one or more of Cu,Ga, and In exhibiting a single phase. The overall composition of thelayers has a composition defined by atomic ratios of ((Cu+Ag)/(In+Ga))<1and (Ga/(In+Ga))<0.5. In some embodiments, the overall composition ofthe layers has a composition defined by atomic ratios of0.7<(Ag+Cu)/(In+Ga)<1.0, 0.05<Ag/(Cu+Ag)<0.3, and 0.0<Ga/(In+Ga)<0.5. Insome embodiments, the plurality of layers comprises one or more sets oftwo or three layers. The depositing is repeated to form a precursor filmof from about 400 nm to about 800 nm in thickness. The chalcogenizingatmosphere comprises one or more of S or Se. After the chalcogenizationstep, the precursor film forms an optical absorber that is from 1.0 μmto 2.5 μm in thickness and has a bandgap between about 1.0 eV and about1.6 eV. The material in the precursor layers remains substantiallyconstant in composition both laterally and throughout the thickness ofeach layer when heated above 155° C., preferably up to about 350° C.

In some embodiments, the depositing is performed using physical vapordeposition (PVD). In some embodiments, one or more sets of three layersare deposited using three PVD targets, wherein one PVD target comprisesAg and In having the following compositions: 1) containing less than 21%In by weight, 2) 26-35 wt % In, or 3) AgIn₂. A second PVD targetcomprises Cu and Ga, and a third PVD target comprises In. In someembodiments, the Cu and Ga target comprises less than 45% Ga by weight.The methods can further comprise grading the bandgap of the absorberlayer by varying the atomic ratio of Ag to (Cu+Ag) through the thicknessof the precursor film. In some embodiments, the deposition using the PVDtarget comprising In is performed in the presence of a chalcogen. Insome embodiments, the deposition using the PVD target is performed in aninert atmosphere and the target comprising In comprises In₂Se₃ or In₂S₃.

In some embodiments, one or more sets of two layers are deposited usingtwo PVD targets, wherein one PVD target comprises Cu₂(In_(x)Ga_(1−x)),x=0.25, and one PVD target comprises In. In some embodiments, one ormore sets of two layers are deposited using two PVD targets, wherein onePVD target comprises Cu(In,Ga) having an atomic ratio of Cu to (In+Ga)greater than 2 and an atomic ratio of Ga to (Ga+In) greater than 0.5,and wherein one PVD target comprises In, In₂Se₃ or In₂S₃.

In some embodiments, one or more sets of three layers are depositedusing three PVD targets, wherein the three PVD targets are selected fromthe following: 1) one PVD target consists essentially of Cu and Ga, onePVD target comprises Cu, and one PVD target comprises In₂Se₃ and In₂S₃,wherein the layers are deposited in an inert atmosphere; or 2) one PVDtarget consists essentially of Cu and Ga, one PVD target comprises Cu,and one PVD target comprises In, wherein the layers comprising Cu and Gaand Cu are deposited in an inert atmosphere, and the layer comprising Inis deposited in an atmosphere comprising one or more of S and Se. Themethods can further comprise grading the bandgap of the absorber layerby varying a ratio of Ga to (Ga+In) through a thickness of the pluralityof precursor layers after the heating. The methods can further comprisegrading a bandgap of the absorber layer by varying a ratio of S to(S+Se) through a thickness of the plurality of precursor layers afterthe heating.

In some embodiments, methods of forming an optical absorber comprisedesignating a plurality of site-isolated regions (SIRs) on thesubstrate, depositing a plurality of layers comprising one or more ofCu, Ga, and In on a substrate using PVD to form a precursor film,heating the layers in a chalcogenizing atmosphere to effect achalcogenization reaction, varying process parameters (e.g., annealingor PVD process parameters) among the plurality of SIRs in acombinatorial manner, and characterizing each precursor film or opticalabsorber formed on the discrete SIRs. In some embodiments, the processparameters comprise one or more of wt % In in Ag—In target, wt % Ga inCu—Ga target, wt % Cu in Cu—In—Ga target, wt % Ga in Cu—In—Ga target, wt% Cu, Ag, In, and Ga in deposited film or stack of layers, sputteringpower, sputtering pressure, sputtering atmosphere composition (e.g., O₂,H₂Se or H₂ in addition to Ar or other noble gas), sputtering time,substrate temperature, annealing temperature and time, annealingatmosphere composition, annealing pressure, number of sets of layers,and co-deposition vs. sequential layer deposition. In some embodiments,the characterizing each precursor film or optical absorber comprisesmeasuring a structure or performance parameter of the precursor film oroptical absorber for each of the plurality of site-isolated regions. Insome embodiments, the structure or performance parameter is one or moreof crystallinity, grain size (distribution), lattice parameter, crystalorientation (distribution), matrix and minority composition, bandgap,bandgap grading, bulk bandgap, surface bandgap, efficiency, resistivity,carrier concentration, mobility, minority carrier lifetime, opticalabsorption coefficient, surface roughness, adhesion, thermal expansioncoefficient, thickness, photoluminescence properties, surfacephotovoltage properties, haze, gloss, specular reflection, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for implementing combinatorial processingand evaluation.

FIG. 2 is a schematic diagram for illustrating various process sequencesusing combinatorial processing and evaluation.

FIG. 3 illustrates a schematic diagram of a simple substrate thin filmphotovoltaic (TFPV) stack according to an embodiment described herein.

FIG. 4 illustrates a schematic diagram of a simple superstrate TFPVstack according to an embodiment described herein.

FIG. 5 illustrates a schematic diagram of a simple superstrate TFPVstack according to an embodiment described herein.

FIG. 6 is a flow chart for a generic 2-step process to form a CIGSeabsorber layer.

FIG. 7 is a flow chart for a generic 4-step process to form a CIGSeabsorber layer.

FIG. 8 is a schematic of an in-line deposition system according to someembodiments.

FIG. 9 is a binary phase diagram for In and Ag.

FIG. 10 is ternary phase diagram for In, Ga and Cu.

FIG. 11 is a flow chart for a method used to form an absorber materialaccording to some embodiments.

FIG. 12 is a flow chart for a method used to form an absorber materialaccording to some embodiments.

FIG. 13 is a flow chart for a method used to form an absorber materialaccording to some embodiments.

FIG. 14 is a flow chart for a method used to form an absorber materialaccording to some embodiments.

DETAILED DESCRIPTION

Before the present invention is described in detail, it is to beunderstood that unless otherwise indicated this invention is not limitedto specific semiconductor devices or to specific semiconductormaterials. Exemplary embodiments will be described for solar cells, butother devices can also be fabricated using the methods disclosed. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tolimit the scope of the present invention.

It must be noted that as used herein and in the claims, the singularforms “a,” “and” and “the” include plural referents unless the contextclearly dictates otherwise. Thus, for example, reference to “a layer”includes two or more layers, and so forth.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the invention. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges, and are also encompassed within the invention, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the invention. Wherethe modifier “about” or “approximately” is used, the stated quantity canvary by up to 10%. Where the modifier “substantially equal to” is used,the two quantities may vary from each other by no more than 5%.

DEFINITIONS

As used herein, the term “thermodynamically stable” or “single phase”will be understood to mean that the material in the precursor layersremains substantially constant in composition both laterally andthroughout the thickness of each layer when heated up to and above 155°C., preferably up to about 350° C. The stability of the stack of layersis dictated by the phase diagram of the elements involved, not thestability of one layer by itself. For example, a stack ofCu₂(In_(0.25),Ga_(0.75))+In is thermodynamically stable up to severalhundred degrees. When a precursor layer comprises pure In (whichexhibits a phase transition [solid to liquid] at 156° C.), the stack ofprecursor layers remains substantially constant in composition, eventhough the In layer melts at 156° C. Therefore, the phases are“thermodynamically stable” as a stack of layers having a constantcomposition in a temperature range (e.g., 25° C. to 350° C.). Inaddition, the composition is uniform laterally, in contrast to layersdeposited using a mixture of particles or flakes, which will exhibitheterogeneity in orientation and composition. The composition in theprecursor layers exhibits a single phase in metals (e.g., an In layer,or a single-phase Cu—Ga alloy layer), but can also exhibit an additionalphase when the layer comprises an alkali-salt. The alkali source can bea component of the single metal phase (e.g., a single-phase In—Nacompound), or a separate salt phase (e.g., NaF) embedded in thesingle-phase metal precursor layer (e.g. In, or single-phase Cu—Ga).Similarly, when a layer in a stack of precursor layers is deposited as achalcogenide (rather than a metal), it exhibits a single phase.

As used herein, “CIGS” will be understood to represent the entire rangeof related alloys denoted byCu_(z)In_((1−x))Ga_(x)S_((2+w)(1−y))Se_((2+w)y), where 0.5≦z≦1.5, 0≦x≦1,0≦y≦1, −0.2≦w≦0.5. Similarly, as noted above, other materials (i.e. Ag,Au, Te, etc.) may be incorporated into potential absorber layers, (withe.g. Ag replacing part or all of the Cu, and Te replacing part or all ofthe Se and/or S). Also as mentioned previously, any of these materialsmay be further doped with a suitable dopant. As used herein, “CIGSSe”,“CIGSe”, and “CIGS” will be defined as equivalent and will be usedinterchangeably and will include all compositions includingCu—In—Ga—Se—S, Cu—In—Ga—Se, and Cu—In—Ga—S. Furthermore, “CIGS” alsoincludes other IB-IIIA-VIA alloys, like (Ag,Cu)(In,Ga)(Se), or(Cu)(In,Ga)(S,Se,Te), and the like.

As used herein, the notation “(IIIA)” will be understood to representthe sum of the concentrations of all Group-IIIA elements. This notationwill be used herein in calculations of the composition ratios of variouselements. This notation will be understood to extend to each of theother Groups of the periodic table respectively (e.g. “(IA)”, “(IIA)”,“(IVA)”, “(VIA)”, “(IB)”, “(IIB)”, etc.).

As used herein, the notation “Cu—In—Ga” and “Cu(In, Ga)” will beunderstood to include a material containing these elements in any ratio.The notation is extendable to other materials and other elementalcombinations.

As used herein, the notation “Cu_(x)In_(y)Ga_(z)” will be understood toinclude a material containing these elements in a specific ratio givenby x, y, and z (e.g. Cu₇₅Ga₂₅ contains 75 atomic % Cu and 25 atomic %Ga). The notation is extendable to other materials and other elementalcombinations.

As used herein, the notation “(Ag,Cu)_(x)(In,Ga)_(y)(Se,S,Te)_(z)” willbe understood to include a material containing a total amount ofGroup-IB elements (i.e. Ag plus Cu, etc.) in a relative amount given byx, a total amount of Group-IIIA elements (i.e. In plus Ga), etc. in arelative amount given by y, and a total amount of Group-VIA elements(i.e. Se plus S plus Te, etc.) in a relative amount given by z. Thenotation is extendable to other materials and other elementalcombinations.

As used herein, “metal chalcogenide” or “chalcogenide” will beunderstood to represent the entire range of related compounds denoted by“MX” where M represents one or more metal elements and X represents oneor more of the chalcogen elements (e.g. O, S, Se, or Te).

As used herein, “chalcogenize” and “chalcogenization” will be understoodto represent the process by which one or more metals are converted tochalcogenide compounds by exposing the one or more metals to a chalcogen(e.g. O, S, Se, or Te) at elevated temperature (e.g. between 100° C. and700° C.). Specifically, “selenization” will be understood to representthe process by which one or more metals are converted to selenidecompounds by exposing the one or more metals to a Se source at elevatedtemperature (e.g. between 100° C. and 700° C.). Specifically,“sulfurization” will be understood to represent the process by which oneor more metals are converted to sulfide compounds by exposing the one ormore metals to a S source at elevated temperature (e.g. between 100° C.and 700° C.). In addition, “chalcogenize” or “chalcogenization” will beunderstood to represent the process by which a metal precursor is eitherpartially or completely converted to the final multinary chalcogenidecompound(s). Similarly, “chalcogenize” or “chalcogenization” will beunderstood to represent the process by which a precursor containing oneor more chalcogenide materials with/without one or more elemental oralloy metals is converted to one or more dense, polycrystalline, desiredmultinary chalcogenide compound(s). It should be understood that themajority of the final film contains the desired multinary chalcogenidecompound(s), yet a minority of the material might not be converted tothe desired multinary chalcogenide compound(s).

As used herein, the terms “film” and “layer” will be understood torepresent a portion of a stack. They will be understood to cover both asingle layer as well as a multilayered structure (i.e. a nanolaminate).As used herein, these terms will be used synonymously and will beconsidered equivalent.

As used herein, “single grading” and “single gradient” will beunderstood to describe cases wherein a parameter varies throughout thethickness of a film or layer and further exhibits a smooth, quasilinearvariation. Examples of suitable parameters used herein will include theatomic concentration of a specific elemental species (i.e. compositionvariation) throughout the thickness of a film or layer, and bandgapenergy variation throughout the thickness of a film or layer.

As used herein, “double grading” and “double gradient” will beunderstood to describe cases wherein a parameter varies throughout thethickness of a film or layer and further exhibits a variation whereinthe value of the parameter is smaller toward the middle of the film orlayer with respect to either end of the film or layer. It is not arequirement that the value of the parameter be equivalent at the twoends of the film or layer. Examples of suitable parameters used hereinwill include the atomic concentration of a specific elemental species(i.e. composition variation) throughout the thickness of a film orlayer, and bandgap energy variation throughout the thickness of a filmor layer.

As used herein, “substrate configuration” will be understood to describecases wherein the TFPV stack is built sequentially on top of a substrateand the light is assumed to be incident upon the top of the TFPV stack.As used herein, an “n-substrate” configuration will be used to denotethat the n-type layer (i.e. buffer layer) is closest to the incidentlight. The n-substrate configuration is the most common. As used herein,a “p-substrate” configuration will be used to denote that the p-typelayer (i.e. absorber layer) is closest to the incident light.

As used herein, “superstrate configuration” will be understood todescribe cases wherein the substrate faces the incident sunlight. Theconvention will be used wherein light is assumed to be incident upon thesubstrate. As used herein, an “n-superstrate” configuration will be usedto denote that the n-type layer (i.e. buffer layer) is closest to theincident light. As used herein, a “p-superstrate” configuration will beused to denote that the p-type layer (i.e. absorber layer) is closest tothe incident light.

As used herein, “substrate” will be understood to generally be one offloat glass, low-iron glass, borosilicate glass, flexible glass,specialty glass for high temperature processing, stainless steel, carbonsteel, aluminum, copper, titanium, molybdenum, polyimide, plastics,cladded metal foils, etc. Furthermore, the substrates may be processedin many configurations such as single substrate processing, multiplesubstrate batch processing, in-line continuous processing, roll-to-rollprocessing, etc. in all of the methods and examples described herein.

As used herein, “precursor layer”, “precursor material”, “metalprecursor layer”, “metal precursor material”, etc. will be understood tobe equivalent and be understood to refer to a metal, metal alloy, metalchalcogenide, etc. layer and/or material that is first deposited andwill ultimately become the absorber layer of the TFPV device after fullchalcogenization and/or further processing.

As used herein, “absorber layer”, “absorber material”, and “opticalabsorber” etc. will be understood to be equivalent and be understood torefer to a layer and/or material that is responsible for the chargegeneration in the TFPV device after full chalcogenization and/or furtherprocessing.

As used herein, the notations “Al:ZnO” and “ZnO:Al” will be understoodto be equivalent and will describe a material wherein the base materialis the metal oxide and the element separated by the colon, “:”, isconsidered a dopant. In this example, Al is a dopant in a base materialof zinc oxide. The notation is extendable to other materials and otherelemental combinations.

As used herein, a “bandgap-increasing metal” will be understood to be ametal element that increases the bandgap when substituted for an elementfrom the same periodic table Group in the absorber material. Forexample, substituting Ag for a portion of the Cu in a CIGS material willincrease the bandgap. For example, increasing the relative amount of Gaversus indium in a CIGS material will increase the bandgap. For example,increasing the relative amount of S versus Se in a CIGS material willincrease the bandgap.

In various FIGs. below, a TFPV material stack is illustrated using asimple planar structure. Those skilled in the art will appreciate thatthe description and teachings to follow can be readily applied to anysimple or complex TFPV solar cell structure (e.g. a stack with(non-)conformal non-planar layers for optimized photon management). Thedrawings are for illustrative purposes only and do not limit theapplication of the present invention.

“Double grading” the bandgap of the CIGS absorber is a method known inthe art to increase the efficiency of CIGS solar cells. In a CIGSabsorber layer that has a double-graded bandgap profile, the bandgap ofthe CIGS layer increases toward the front surface and toward the backsurface of the CIGS layer, with a bandgap minimum located in a centerregion of the CIGS layer. Double grading helps in reducing unwantedcharge carrier recombination. The increasing bandgap profile at the backsurface of the CIGS layer, (i.e., the absorber surface that is remotefrom the incident light in the substrate configuration), creates a backsurface field, which reduces recombination at the back surface andenhances carrier collection. Generally, in the disclosure to follow, thedescription will apply to the “n-substrate” configuration for economy oflanguage. However, those skilled in the art will understand that thedisclosure is also equally applicable to either of the “p-substrate” or“n, p-superstrate” configurations discussed previously.

The efficiency of thin-film photovoltaic (TFPV) devices depends on manyproperties of the absorber layer and the buffer layer such ascrystallinity, grain size, composition uniformity, density, defectconcentration, doping level, surface roughness, etc.

The manufacture of TFPV devices entails the integration and sequencingof many unit processing steps. As an example, TFPV manufacturingtypically includes a series of processing steps such as cleaning,surface preparation, deposition, patterning, etching, thermal annealing,and other related unit processing steps. The precise sequencing andintegration of the unit processing steps enables the formation offunctional devices meeting desired performance metrics such asefficiency, power production, and reliability.

The development of TFPV devices exploiting polycrystalline, multinarycompound semiconductors represents a daunting challenge in terms of thetime-to-commercialization. That same development also suggests anenticing opportunity for breakthrough discoveries. A quaternary systemsuch as CIGS requires management of multiple kinetic pathways,thermodynamic phase equilibrium considerations, defect chemistries, andinterfacial control. The vast phase-space to be managed includes processparameters, source material choices, compositions, and overallintegration schemes. The complexity of the intrinsically-doped,self-compensating, multinary, polycrystalline, queue-time-sensitive,thin-film absorber (CIGS), and its interfaces to up-, and down-streamprocessing, combined with the lack of knowledge on a device level toaddress efficiency losses effectively, makes it a highly empiricalmaterial system. The performance of any thin-film,(opto-)electronically-active device is extremely sensitive to itsinterfaces. Interface engineering for electronically-active devices ishighly empirical. Traditional R&D methods are ill-equipped to addresssuch complexity, and the traditionally slow pace of R&D could limit anynew material from reaching industrial relevance when having to competewith the incrementally improving performance of already established TFPVfabrication lines, and continuously decreasing panel prices for moretraditional cSi PV technologies.

Due to the complexity of the material, cell structure, and manufacturingprocess, both the fundamental scientific understanding and large scalemanufacturability are yet to be realized for TFPV devices. As thephotovoltaic industry pushes to achieve grid parity, much faster andbroader investigation is needed to explore the material, device, andprocess windows for higher efficiency and a lower cost of manufacturingprocess. Efficient methods for forming different types of TFPV devicesthat can be evaluated are necessary.

In light of the above, there is a need in the art for an economicalmethod of creating CIGS absorber layers having a graded bandgap andhigher efficiencies. Improved layer quality and a graded bandgap enablehigher efficiency CIGS solar cells to be made.

As part of the discovery, optimization and qualification of each unitprocess, it is desirable to be able to i) test different materials, ii)test different processing conditions within each unit process module,iii) test different sequencing and integration of processing moduleswithin an integrated processing tool, iv) test different sequencing ofprocessing tools in executing different process sequence integrationflows, and combinations thereof in the manufacture of devices such asTFPV devices. In particular, there is a need to be able to test i) morethan one material, ii) more than one processing condition, iii) morethan one sequence of processing conditions, iv) more than one processsequence integration flow, and combinations thereof, collectively knownas “combinatorial process sequence integration”, on a single substratewithout the need of consuming the equivalent number of monolithicsubstrates per material(s), processing condition(s), sequence(s) ofprocessing conditions, sequence(s) of processes, and combinationsthereof. This can greatly improve both the speed and reduce the costsassociated with the discovery, implementation, optimization, andqualification of material(s), process(es), and process integrationsequence(s) required for manufacturing.

Systems and methods for High Productivity Combinatorial (HPC) processingare described in U.S. Pat. No. 7,544,574 filed on Feb. 10, 2006, U.S.Pat. No. 7,824,935 filed on Jul. 2, 2008, U.S. Pat. No. 7,871,928 filedon May 4, 2009, U.S. Pat. No. 7,902,063 filed on Feb. 10, 2006, and U.S.Pat. No. 7,947,531 filed on Aug. 28, 2009 which are all hereinincorporated by reference. Systems and methods for HPC processing arefurther described in U.S. patent application Ser. No. 11/352,077 filedon Feb. 10, 2006, claiming priority from Oct. 15, 2005, U.S. patentapplication Ser. No. 11/419,174 filed on May 18, 2006, claiming priorityfrom Oct. 15, 2005, U.S. patent application Ser. No. 11/674,132 filed onFeb. 12, 2007, claiming priority from Oct. 15, 2005, and U.S. patentapplication Ser. No. 11/674,137 filed on Feb. 12, 2007, claimingpriority from Oct. 15, 2005 which are all herein incorporated byreference.

HPC processing techniques have been successfully adapted to wet chemicalprocessing such as etching, texturing, polishing, cleaning, etc. HPCprocessing techniques have also been successfully adapted to depositionprocesses such as sputtering, atomic layer deposition (ALD), andchemical vapor deposition (CVD).

HPC processing techniques have been adapted to the development andinvestigation of absorber layers and buffer layers for TFPV solar cellsas described in U.S. application Ser. No. 13/236,430 filed on Sep. 19,2011, entitled “COMBINATORIAL METHODS FOR DEVELOPING SUPERSTRATE THINFILM SOLAR CELLS” and is incorporated herein by reference. However, HPCprocessing techniques have not been successfully adapted to thedevelopment of contact structures for TFPV devices. Generally, there aretwo basic configurations for TFPV devices. The first configuration isknown as a “substrate” configuration. In this configuration, the contactthat is formed on or near the substrate is called the back contact. Inthis configuration, the light is incident on the TFPV device from thetop of the material stack (i.e. the side opposite the substrate). CIGSTFPV devices are most commonly manufactured in this configuration. Thesecond configuration is known as a “superstrate” configuration. In thisconfiguration, the contact that is formed on or near the substrate iscalled the front contact. In this configuration, the light is incidenton the TFPV device through the substrate. CdTe, and a-Si, TFPV devicesare most commonly manufactured in this configuration. In bothconfigurations, light trapping schemes may be implemented in the contactlayer that is formed on or near the substrate. Additionally, otherefficiency or durability improvements can be implemented in the contactlayer that is formed farthest away from the substrate.

FIG. 1 illustrates a schematic diagram, 100, for implementingcombinatorial processing and evaluation using primary, secondary, andtertiary screening. The schematic diagram, 100, illustrates that therelative number of combinatorial processes run with a group ofsubstrates decreases as certain materials and/or processes are selected.Generally, combinatorial processing includes performing a large numberof processes during a primary screen, selecting promising candidatesfrom those processes, performing the selected processing during asecondary screen, selecting promising candidates from the secondaryscreen for a tertiary screen, and so on. In addition, feedback fromlater stages to earlier stages can be used to refine the successcriteria and provide better screening results.

For example, thousands of materials are evaluated during a materialsdiscovery stage, 102. Materials discovery stage, 102, is also known as aprimary screening stage performed using primary screening techniques.Primary screening techniques may include dividing substrates intocoupons and depositing materials using varied processes. The materialsare then evaluated, and promising candidates are advanced to thesecondary screen, or materials and process development stage, 104.Evaluation of the materials is performed using metrology tools such aselectronic testers and imaging tools (i.e., microscopes).

The materials and process development stage, 104, may evaluate hundredsof materials (i.e., a magnitude smaller than the primary stage) and mayfocus on the processes used to deposit or develop those materials.Promising materials and processes are again selected, and advanced tothe tertiary screen or process integration stage, 106, where tens ofmaterials and/or processes and combinations are evaluated. The tertiaryscreen or process integration stage, 106, may focus on integrating theselected processes and materials with other processes and materials.

The most promising materials and processes from the tertiary screen areadvanced to device qualification, 108. In device qualification, thematerials and processes selected are evaluated for high volumemanufacturing, which normally is conducted on full substrates withinproduction tools, but need not be conducted in such a manner. Theresults are evaluated to determine the efficacy of the selectedmaterials and processes. If successful, the use of the screenedmaterials and processes can proceed to pilot manufacturing, 110.

The schematic diagram, 100, is an example of various techniques that maybe used to evaluate and select materials and processes for thedevelopment of new materials and processes. The descriptions of primary,secondary, etc. screening and the various stages, 102-110, are arbitraryand the stages may overlap, occur out of sequence, be described and beperformed in many other ways.

This application benefits from High Productivity Combinatorial (HPC)techniques described in U.S. patent application Ser. No. 11/674,137filed on Feb. 12, 2007 which is hereby incorporated for reference in itsentirety. Portions of the '137 application have been reproduced below toenhance the understanding of the present invention. The embodimentsdescribed herein enable the application of combinatorial techniques toprocess sequence integration in order to arrive at a globally optimalsequence of TFPV manufacturing operations by considering interactioneffects between the unit manufacturing operations, the processconditions used to effect such unit manufacturing operations, hardwaredetails used during the processing, as well as materials characteristicsof components utilized within the unit manufacturing operations. Ratherthan only considering a series of local optimums, i.e., where the bestconditions and materials for each manufacturing unit operation isconsidered in isolation, the embodiments described below considerinteractions effects introduced due to the multitude of processingoperations that are performed and the order in which such multitude ofprocessing operations are performed when fabricating a TFPV device. Aglobal optimum sequence order is therefore derived and as part of thisderivation, the unit processes, unit process parameters and materialsused in the unit process operations of the optimum sequence order arealso considered.

The embodiments described further analyze a portion or sub-set of theoverall process sequence used to manufacture a TFPV device. Once thesubset of the process sequence is identified for analysis, combinatorialprocess sequence integration testing is performed to optimize thematerials, unit processes, hardware details, and process sequence usedto build that portion of the device or structure. During the processingof some embodiments described herein, structures are formed on theprocessed substrate that are equivalent to the structures formed duringactual production of the TFPV device. For example, such structures mayinclude, but would not be limited to, contact layers, buffer layers,absorber layers, or any other series of layers or unit processes thatcreate an intermediate structure found on TFPV devices. While thecombinatorial processing varies certain materials, unit processes,hardware details, or process sequences, the composition or thickness ofthe layers or structures or the action of the unit process, such ascleaning, surface preparation, deposition, surface treatment, etc. issubstantially uniform through each discrete region. Furthermore, whiledifferent materials or unit processes may be used for correspondinglayers or steps in the formation of a structure in different regions ofthe substrate during the combinatorial processing, the application ofeach layer or use of a given unit process is substantially consistent oruniform throughout the different regions in which it is intentionallyapplied. Thus, the processing is uniform within a region (inter-regionuniformity) and between regions (intra-region uniformity), as desired.It should be noted that the process can be varied between regions, forexample, where a thickness of a layer is varied or a material may bevaried between the regions, etc., as desired by the design of theexperiment.

The result is a series of regions on the substrate that containstructures or unit process sequences that have been uniformly appliedwithin that region and, as applicable, across different regions. Thisprocess uniformity allows comparison of the properties within and acrossthe different regions such that the variations in test results are dueto the varied parameter (e.g., materials, unit processes, unit processparameters, hardware details, or process sequences) and not the lack ofprocess uniformity. In the embodiments described herein, the positionsof the discrete regions on the substrate can be defined as needed, butare preferably systematized for ease of tooling and design ofexperimentation. In addition, the number, variants and location ofstructures within each region are designed to enable valid statisticalanalysis of the test results within each region and across regions to beperformed.

FIG. 2 is a simplified schematic diagram illustrating a generalmethodology for combinatorial process sequence integration that includessite isolated processing and/or conventional processing in accordancewith one embodiment of the invention. In one embodiment, the substrateis initially processed using conventional process N. In one exemplaryembodiment, the substrate is then processed using site isolated processN+1. During site isolated processing, an HPC module may be used, such asthe HPC module described in U.S. patent application Ser. No. 11/352,077filed on Feb. 10, 2006. The substrate can then be processed using siteisolated process N+2, and thereafter processed using conventionalprocess N+3. Testing is performed and the results are evaluated. Thetesting can include physical, chemical, acoustic, magnetic, electrical,optical, etc. tests. From this evaluation, a particular process from thevarious site isolated processes (e.g. from steps N+1 and N+2) may beselected and fixed so that additional combinatorial process sequenceintegration may be performed using site isolated processing for eitherprocess N or N+3. For example, a next process sequence can includeprocessing the substrate using site isolated process N, conventionalprocessing for processes N+1, N+2, and N+3, with testing performedthereafter.

It should be appreciated that various other combinations of conventionaland combinatorial processes can be included in the processing sequencewith regard to FIG. 2. That is, the combinatorial process sequenceintegration can be applied to any desired segments and/or portions of anoverall process flow. The combinatorial processing may employ uniformprocessing of site isolated regions or may employ gradient techniques.Characterization, including physical, chemical, acoustic, magnetic,electrical, optical, etc. testing, can be performed after each processoperation, and/or series of process operations within the process flowas desired. The feedback provided by the testing is used to selectcertain materials, processes, process conditions, and process sequencesand eliminate others. Furthermore, the above flows can be applied toentire monolithic substrates, or portions of monolithic substrates suchas coupons.

Under combinatorial processing operations the processing conditions atdifferent regions can be controlled independently. Consequently, processmaterial amounts, reactant species, processing temperatures, processingtimes, processing pressures, processing flow rates, processing powers,processing reagent compositions, the rates at which the reactions arequenched, deposition order of process materials, process sequence steps,hardware details, etc., can be varied from region to region on thesubstrate. Thus, for example, when exploring materials, a processingmaterial delivered to a first and second region can be the same ordifferent. If the processing material delivered to the first region isthe same as the processing material delivered to the second region, thisprocessing material can be offered to the first and second regions onthe substrate at different concentrations. In addition, the material canbe deposited under different processing parameters. Parameters which canbe varied include, but are not limited to, process material amounts,reactant species, processing temperatures, processing times, processingpressures, processing flow rates, processing powers, processing reagentcompositions, the rates at which the reactions are quenched, atmospheresin which the processes are conducted, an order in which materials aredeposited, hardware details of the gas distribution assembly, etc. Itshould be appreciated that these process parameters are exemplary andnot meant to be an exhaustive list as other process parameters commonlyused in TFPV manufacturing may be varied.

As mentioned above, within a region, the process conditions aresubstantially uniform. That is, the embodiments, described hereinlocally perform the processing in a conventional manner, e.g.,substantially consistent and substantially uniform, while globally overthe substrate, the materials, processes, and process sequences may vary.Thus, the testing will find optimums without interference from processvariation differences between processes that are meant to be the same.However, in some embodiments, the processing may result in a gradientwithin the regions. It should be appreciated that a region may beadjacent to another region in one embodiment or the regions may beisolated and, therefore, non-overlapping. When the regions are adjacent,there may be a slight overlap wherein the materials or precise processinteractions are not known, however, a portion of the regions, normallyat least 50% or more of the area, is uniform and all testing occurswithin that region. Further, the potential overlap is only allowed withmaterial of processes that will not adversely affect the result of thetests. Both types of regions are referred to herein as regions ordiscrete regions.

FIG. 3 illustrates a schematic diagram of a simple TFPV device stack inthe substrate configuration consistent with some embodiments of thepresent invention. The convention will be used wherein light is assumedto be incident upon the top of the material stack in the substrateconfiguration as illustrated. This generic diagram would be typical of aCIGS TFPV device. A back contact layer, 304, is formed on a substrate,302. Examples of suitable substrates comprise float glass, low-ironglass, borosilicate glass, flexible glass, specialty glass for hightemperature processing, stainless steel, carbon steel, aluminum, copper,titanium, molybdenum, polyimide, plastics, cladded metal foils, etc.Furthermore, the substrates may be processed in many configurations suchas single substrate processing, multiple substrate batch processing,in-line continuous processing, roll-to-roll processing, etc. As usedherein, the phrase “back contact” will be understood to be the primarycurrent conductor layer situated between the substrate and the absorberlayer in a substrate configuration TFPV device. An example of a commonback contact layer material is Mo for CIGS TFPV devices. Other types ofTFPV devices use different materials for the back contact. As anexample, Cu-based materials such as Cu/Au, Cu/graphite, Cu/Mo,Cu:ZnTe/Mo, etc. are typically used for CdTe TFPV devices andtransparent conductive oxide (TCO) materials such as ZnO, ITO, SnO₂:F,etc. are typically used for a-Si TFPV devices. The back contact layermay be formed by any number of deposition technologies. Examples ofsuitable deposition technologies comprise physical vapor deposition(PVD) (e.g. sputtering), evaporation, chemical vapor deposition (CVD),atomic layer deposition (ALD), plating, printing, wet coating, etc. Thethickness of the back contact layer is typically between about 0.3 umand about 1.0 um. The back contact layer has a number of requirementssuch as high conductivity, good ohmic contact to the absorber layer,ease of bonding to tabs for external connectivity, ease of scribing orother removal, good thermo-mechanical stability, and chemical resistanceduring subsequent processing, among others.

Optionally, a diffusion barrier and/or adhesion-promotion layer (notshown) may be formed between the substrate and the back contact layer.When implemented, the diffusion barrier layer stops the diffusion ofimpurities from the substrate into the back contact layer, oralternatively, stops the diffusion and reaction of the back contactmaterial with the substrate. Examples of common diffusion barrier and/oradhesion-promotion layers comprise chromium, vanadium, tungsten,nitrides such as tantalum nitride, tungsten nitride, titanium nitride,silicon nitride, zirconium nitride, hafnium nitride, oxy-nitrides suchas tantalum oxy-nitride, tungsten oxy-nitride, titanium oxy-nitride,silicon oxy-nitride, zirconium oxy-nitride, hafnium oxy-nitride, oxidessuch as aluminum oxide, silicon oxide, carbides such as silicon carbide,binary and/or multinary compounds of tungsten, titanium, molybdenum,chromium, vanadium, tantalum, hafnium, zirconium, and/or niobiumwith/without the inclusion of nitrogen and/or oxygen. The diffusionbarrier layer may be formed, partially or completely, from any wellknown technique such as sputtering, ALD, CVD, evaporation, wet methodssuch as printing or spraying of inks, screen printing, inkjet printing,slot die coating, gravure printing, wet chemical depositions, or fromsol-gel methods, such as the coating, drying, and firing ofpolysilazanes.

A p-type absorber layer, 306, of CIGS is then deposited on top of theback contact layer. The absorber layer may be formed, partially orcompletely, using a variety of techniques such as PVD (sputtering),co-evaporation, in-line evaporation, plating, printing or spraying ofinks, screen printing, inkjet printing, slot die coating, gravureprinting, wet chemical depositions, CVD, etc. Advantageously, theabsorber layer is deficient in Cu. The Cu deficiency may be controlledby managing the deposition conditions. Advantageously, a small amount ofNa is present during the absorber growth. The Na may be added byout-diffusion from the SLG substrate or may be purposely added in theform of Na₂Se, NaF, sodium alloys of In and/or Ga, or another Na source,prior, during, or after the deposition and/or growth of the absorberlayer. Optionally, the absorber layer is deposited as a precursor layerthat undergoes a selenization process after formation to convert theprecursor to CIGS into a high-quality CIGS semiconductor film. Theselenization process involves the exposure of the precursor and/orabsorber layer to H₂Se, H₂S, Se vapor, S vapor, or diethylselenide(DESe) at temperatures most typically between about 300° C. and 700° C.It should be noted that the precursor to CIGS might already contain achalcogen source (e.g. Se), either as a separate layer, or incorporatedinto the bulk of the precursor layer. The precursor film can be a stackof layers, or one layer. The precursor layer can be dense, or porous.The precursor film typically contains Cu, In, and Ga. The precursorlayer is most commonly deposited by sputtering from e.g. binarycopper-gallium and Indium sputter targets. Nevertheless, plating andprinting to deposit the metal precursor film containing Cu, In, and/orGa are used as well. During the selenization process, a layer ofMo(S,Se)₂ (not shown) forms at the back contact/absorber layer interfaceand forms a fairly good ohmic contact between the two layers.Alternatively, a layer of Mo(S,Se)₂ (not shown) can be deposited at theback contact/absorber layer interface using a variety of well knowntechniques such as PVD (sputtering), CBD, ALD, plating, etc. Thethickness of the absorber layer is typically between about 1.0 μm andabout 3.0 μm. The performance of the absorber layer is sensitive tomaterials properties such as crystallinity, grain size, surfaceroughness, composition, defect concentration, etc. as well as processingparameters such as temperature, deposition rate, thermal treatments,etc.

An n-type buffer layer, 308, is then deposited on top of the absorberlayer. Examples of suitable n-type buffer layers comprise CdS, ZnS,In₂S₃, In₂(S,Se)₃, (Cd,Zn)S, ZnO, Zn(O,S), (Zn,Mg)O, etc. CdS is thematerial most often used as the n-type buffer layer in CIGS TFPVdevices. The buffer layer may be deposited using chemical bathdeposition (CBD), chemical surface deposition (CSD), PVD (sputtering),printing, plating, ALD, Ion-Layer-Gas-Reaction (ILGAR), ultrasonicspraying, or evaporation. The thickness of the buffer layer is typicallybetween about 30 nm and about 100 nm. The performance of the bufferlayer is sensitive to materials properties such as crystallinity, grainsize, surface roughness, composition, defect concentration, etc. as wellas processing parameters such as temperature, deposition rate, thermaltreatments, etc.

Optionally, an intrinsic ZnO (iZnO) layer, 310, is then formed on top ofthe buffer layer. The iZnO layer is a high resistivity material andforms part of the transparent conductive oxide (TCO) stack that servesas part of the front contact structure. Other resistive metal oxideslike SnO₂, resistive ZnO:Al, resistive In—Ga—Zn—O, etc. might be usedinstead of i-ZnO. The TCO stack is formed from transparent conductivemetal oxide materials and collects charge across the face of the TFPVsolar cell and conducts the charge to tabs used to connect the solarcell to external loads. The iZnO layer makes the TFPV solar cell lesssensitive to lateral non-uniformities caused by differences incomposition or defect concentration in the absorber and/or bufferlayers. The iZnO layer is typically between about 0 nm and 150 nm inthickness. The iZnO layer is typically formed using a (reactive) PVD(sputtering) technique or CVD technique, but can be deposited by platingor printing as well. A low resistivity top TCO layer, 312, (examplesinclude Al:ZnO (AZO), (In,Sn)O (ITO), (In,Zn)O, B:ZnO, Ga:ZnO, F:ZnO,F:SnO₂, etc.) is formed on top of the iZnO layer. The top TCO layer istypically between about 0.25 um and 1.0 um in thickness. The top TCOlayer is typically formed using a (reactive) PVD (sputtering) techniqueor CVD technique. Optionally, the transparent top electrode can beprinted or wet-coated from (silver) nano-wires, carbon nanotubes, andthe like.

FIG. 4 illustrates a simple CIGS TFPV device material stack, 400,consistent with some embodiments of the present invention. The CIGS TFPVdevice illustrated in FIG. 4 is shown in a superstrate configurationwherein the glass substrate faces the incident sunlight. The conventionwill be used wherein light is assumed to be incident upon the substrateand material stack as illustrated. As used herein, this configurationwill be labeled an “n-superstrate” configuration to denote that then-type layer (i.e. buffer layer) is closest to the incident light. Thislabel is to distinguish the configuration from an alternateconfiguration described with respect to FIG. 5 below. The formation ofthe CIGS TFPV device will be described starting with the substrate.Examples of suitable substrates comprise float glass, low-iron glass,borosilicate glass, flexible glass, specialty glass for high temperatureprocessing, polyimide, plastics, etc. Furthermore, the substrates may beprocessed in many configurations such as single substrate processing,multiple substrate batch processing, in-line continuous processing,roll-to-roll processing, etc.

A low resistivity bottom TCO front contact layer, 404, (examples includeAl:ZnO (AZO), (In,Sn)O (ITO), (In,Zn)O, B:ZnO, Ga:ZnO, F:ZnO, F:SnO₂,etc.) is formed on top of the substrate, 402. As used herein, the phrase“front contact” will be understood to be the primary current conductorlayer situated between the substrate and the buffer layer in asuperstrate configuration TFPV device. The bottom TCO layer is typicallybetween about 0.3 um and 2.0 um in thickness. The bottom TCO layer istypically formed using a reactive PVD (sputtering) technique or CVDtechnique.

Optionally, a diffusion barrier and/or adhesion-promotion layer (notshown) may be formed between the substrate, 402, and the front contactlayer, 404. When implemented, the diffusion barrier layer stops thediffusion of impurities from the substrate into the TCO, oralternatively, stops the diffusion and reaction of the TCO material andabove layers with the substrate. Examples of common diffusion barrierand/or adhesion-promotion layers comprise chromium, vanadium, tungsten,nitrides such as tantalum nitride, tungsten nitride, titanium nitride,silicon nitride, zirconium nitride, hafnium nitride, oxy-nitrides suchas tantalum oxy-nitride, tungsten oxy-nitride, titanium oxy-nitride,silicon oxy-nitride, zirconium oxy-nitride, hafnium oxy-nitride, oxidessuch as aluminum oxide, silicon oxide, carbides such as silicon carbide,binary and/or multinary compounds of tungsten, titanium, molybdenum,chromium, vanadium, tantalum, hafnium, zirconium, and/or niobiumwith/without the inclusion of nitrogen and/or oxygen. It should beunderstood that the diffusion barrier layer composition and thicknessare optimized for optical transparency as necessary for the superstrateconfiguration. The diffusion barrier layer may be formed from any wellknown technique such as sputtering, ALD, CVD, evaporation, wet methodssuch as printing or spraying of inks, screen printing, inkjet printing,slot die coating, gravure printing, wet chemical depositions, or fromsol-gel methods, such as the coating, drying, and firing ofpolysilazanes.

An intrinsic iZnO layer, 406, is then formed on top of the TCO layer.The iZnO layer is a high resistivity material and forms part of thetransparent conductive oxide (TCO) stack that serves as part of thefront contact structure. Other resistive metal oxides like SnO₂,resistive ZnO:Al, resistive In—Ga—Zn—O, etc. might be used instead ofi-ZnO. The iZnO layer makes the TFPV device less sensitive to lateralnon-uniformities caused by differences in composition or defectconcentration in the absorber and/or buffer layers. The iZnO layer istypically between about 0 nm and 150 nm in thickness. The iZnO layer istypically formed using a reactive PVD (sputtering) technique or CVDtechnique.

An n-type buffer layer, 408, is then deposited on top of the iZnO layer,406. Examples of suitable n-type buffer layers comprise CdS, ZnS, In₂S₃,In₂(S,Se)₃, (Cd,Zn)S, ZnO, Zn(O,S), (Zn,Mg)O, etc. CdS is the materialmost often used as the n-type buffer layer in CIGS TFPV devices. Thebuffer layer may be deposited using chemical bath deposition (CBD),chemical surface deposition (CSD), PVD (sputtering), printing, plating,ALD, Ion-Layer-Gas-Reaction (ILGAR), ultrasonic spraying, orevaporation. The thickness of the buffer layer is typically betweenabout 30 nm and about 100 nm. The performance of the buffer layer issensitive to materials properties such as crystallinity, grain size,surface roughness, composition, defect concentration, etc. as well asprocessing parameters such as temperature, deposition rate, thermaltreatments, etc.

A p-type absorber layer, 410, of CIGS is then deposited on top of thebuffer layer. The absorber layer may be formed, partially or completely,using a variety of techniques such as PVD (sputtering), co-evaporation,in-line evaporation, plating, printing or spraying of inks, screenprinting, inkjet printing, slot die coating, gravure printing, wetchemical depositions, CVD, etc. Advantageously, the absorber layer isdeficient in Cu. The Cu deficiency may be controlled by managing thedeposition conditions. Advantageously, a small amount of Na is presentduring the growth of the absorber. The Na may be purposely added in theform of Na₂Se or another Na source, prior, during, or after thedeposition and/or growth of the absorber layer. Optionally, the absorberlayer is deposited as a precursor layer that undergoes a selenizationprocess after formation to convert the precursor to CIGS into ahigh-quality CIGS semiconductor film. The selenization process involvesthe exposure of the precursor and/or absorber layer to H₂Se, H₂S, Sevapor, S vapor, or diethylselenide (DESe) at temperatures most typicallybetween about 300° C. and 700° C. It should be noted that the precursorto CIGS might already contain a chalcogen source (e.g. Se), either as aseparate layer, or incorporated into the bulk of the precursor layer.The precursor film can be a stack of layers, or one layer. The precursorlayer can be dense, or porous. The precursor film typically contains Cu,In, and Ga. The precursor layer is most commonly deposited by sputteringfrom e.g. binary Cu—Ga and In sputter targets. Nevertheless, plating andprinting to deposit the metal precursor film containing Cu, In, and/orGa are used as well. During subsequent processing, a layer of Mo(S,Se)₂(not shown) is formed at the back contact/absorber layer interface andforms a fairly good ohmic contact between the two layers. The thicknessof the absorber layer is typically between about 1.0 um and about 3.0um. The performance of the absorber layer is sensitive to materialsproperties such as crystallinity, grain size, surface roughness,composition, defect concentration, etc. as well as processing parameterssuch as temperature, deposition rate, thermal treatments, etc.

A back contact layer, 412, is formed on absorber layer, 410. An exampleof a common back contact layer material is Mo for CIGS TFPV devices. Theback contact layer may be formed by any number of depositiontechnologies. Examples of suitable deposition technologies comprise PVD(sputtering), evaporation, chemical vapor deposition (CVD), atomic layerdeposition (ALD), plating, etc. The thickness of the back contact layeris typically between about 0.3 um and about 1.0 um. The back contactlayer has a number of requirements such as high conductivity, good ohmiccontact to the absorber layer, ease of bonding to tabs for externalconnectivity, ease of scribing or other removal, good thermo-mechanicalstability, and chemical resistance during subsequent processing, amongothers. Other types of TFPV devices use different materials for the backcontact. As an example, Cu-based materials such as Cu/Au, Cu/graphite,Cu/Mo, Cu:ZnTe/Mo, etc. are typically used for CdTe TFPV devices and TCOmaterials such as ZnO, ITO, SnO₂:F, etc. are typically used for a-SiTFPV devices.

FIG. 5 illustrates a simple CIGS TFPV device material stack, 500,consistent with some embodiments of the present invention. The CIGS TFPVdevice illustrated in FIG. 5 is shown in a superstrate configurationwherein the glass substrate faces the incident sunlight. The conventionwill be used wherein light is assumed to be incident upon the substrateand material stack as illustrated. As used herein, this configurationwill be labeled a “p-superstrate” configuration to denote that thep-type layer (i.e. absorber layer) is closest to the incident light.This label is to distinguish the configuration from the alternateconfiguration described with respect to FIG. 4 previously. The formationof the CIGS TFPV device will be described starting with the substrate.Examples of suitable substrates comprise float glass, low-iron glass,borosilicate glass, flexible glass, specialty glass for high temperatureprocessing, polyimide, plastics, etc. Furthermore, the substrates may beprocessed in many configurations such as single substrate processing,multiple substrate batch processing, in-line continuous processing,roll-to-roll processing, etc.

A low resistivity bottom TCO front contact layer (examples includeAl:ZnO (AZO), (In,Sn)O (ITO), (In,Zn)O, B:ZnO, Ga:ZnO, F:ZnO, F:SnO₂,etc.), 504, is formed on top of the substrate, 502. As used herein, thephrase “front contact” will be understood to be the primary currentconductor layer situated between the substrate and the absorber layer ina superstrate configuration TFPV device. The bottom TCO layer istypically between about 0.3 um and 2.0 um in thickness. The bottom TCOlayer is typically formed using a reactive PVD (sputtering) technique orCVD technique. The TCO can be a p-type TCO, (e.g. ternary-based oxide inthe family of Co₃O₄-based spinels, like Co₂ZnO₄ and Co₂NiO₄).Nevertheless, it should be understood that an n-type TCO with anadditional layer (e.g. a heavily-doped p-type semiconductor layer, orMoSe₂) between the TCO and the absorber can be used as well.Furthermore, the TCO might be a bi- or multi-layer of an n-type TCO incontact with the substrate, followed by an ultrathin metal layer, (e.g.like Ag), followed by a thin p-type TCO in contact with the absorberlayer, with/without an additional MoSe₂ layer between the p-type TCO andthe absorber layer.

Optionally, a diffusion barrier and/or adhesion-promotion layer (notshown) may be formed between the substrate, 502, and the front contactlayer. 504. When implemented, the diffusion barrier and/oradhesion-promotion layer stops the diffusion of impurities from thesubstrate into the TCO, or alternatively, stops the diffusion andreaction of the TCO material and above layers with the substrate.Examples of common diffusion barrier and/or adhesion-promotion layerscomprise chromium, vanadium, tungsten, nitrides such as tantalumnitride, tungsten nitride, titanium nitride, silicon nitride, zirconiumnitride, hafnium nitride, oxy-nitrides such as tantalum oxy-nitride,tungsten oxy-nitride, titanium oxy-nitride, silicon oxy-nitride,zirconium oxy-nitride, hafnium oxy-nitride, oxides such as aluminumoxide, silicon oxide, carbides such as silicon carbide, binary and/ormultinary compounds of tungsten, titanium, molybdenum, chromium,vanadium, tantalum, hafnium, zirconium, and/or niobium with/without theinclusion of nitrogen and/or oxygen. It should be understood that thediffusion barrier and/or adhesion-promotion layer composition andthickness are optimized for optical transparency as necessary for thesuperstrate configuration. The diffusion barrier and/oradhesion-promotion layer may be formed from any well known techniquesuch as sputtering, ALD, CVD, evaporation, wet methods such as printingor spraying of inks, screen printing, inkjet printing, slot die coating,gravure printing, wet chemical depositions, or from sol-gel methods suchas the coating, drying, and firing of polysilazanes.

A p-type absorber layer, 506, of CIGS is then deposited on top of thefront contact layer. The absorber layer may be formed, partially, orcompletely, using a variety of techniques such as PVD (sputtering),co-evaporation, in-line evaporation, plating, printing or spraying ofinks, screen printing, inkjet printing, slot die coating, gravureprinting, wet chemical depositions, CVD, etc. Advantageously, theabsorber layer is deficient in Cu. The Cu deficiency may be controlledby managing the deposition conditions. Advantageously, a small amount ofNa is present during the growth of the absorber. The Na may be purposelyadded in the form of Na₂Se or another Na source, prior, during, or afterthe deposition of the precursor and/or absorber layer. Typically, theabsorber layer is deposited as a precursor layer that undergoes achalcogenization (e.g. selenization) process after formation to convertthe precursor to CIGS into a high-quality CIGS semiconductor film. Thechalcogenization process involves the exposure of the precursor and/orabsorber layer to H₂Se, H₂S, Se vapor, S vapor, or diethylselenide(DESe) at temperatures most typically between about 300° C. and 700° C.It should be noted that the precursor to CIGS might already contain achalcogen source (e.g. Se), either as a separate layer, or incorporatedinto the bulk of the precursor layer. The precursor film can be a stackof layers, or one layer. The precursor layer can be dense, or porous.The precursor film typically contains Cu, In, and Ga. The precursorlayer is most commonly deposited by sputtering from e.g. binarycopper-gallium and Indium sputter targets. Nevertheless, plating andprinting to deposit the metal precursor film containing Cu, In, and/orGa are used as well. The thickness of the absorber layer is typicallybetween about 1.0 μm and about 3.0 μm. The performance of the absorberlayer is sensitive to materials properties such as crystallinity, grainsize, surface roughness, composition, defect concentration, etc. as wellas processing parameters such as temperature, deposition rate, thermaltreatments, etc.

An n-type buffer layer, 508, is then deposited on top of the absorberlayer. Examples of suitable n-type buffer layers comprise CdS, ZnS,In₂S₃, In₂(S,Se)₃, (Cd,Zn)S, ZnO, Zn(O,S), (Zn,Mg)O, etc. CdS is thematerial most often used as the n-type buffer layer in CIGS TFPVdevices. The buffer layer may be deposited using chemical bathdeposition (CBD), chemical surface deposition (CSD), PVD (sputtering),printing, plating, ALD, Ion-Layer-Gas-Reaction (ILGAR), ultrasonicspraying, or evaporation. The thickness of the buffer layer is typicallybetween about 30 nm and about 100 nm. The performance of the bufferlayer is sensitive to materials properties such as crystallinity, grainsize, surface roughness, composition, defect concentration, etc. as wellas processing parameters such as temperature, deposition rate, thermaltreatments, etc.

An intrinsic iZnO layer, 510, is then formed on top of the buffer layer.The iZnO layer is a high resistivity material and forms part of the backcontact structure. Other resistive metal oxides like SnO₂, resistiveZnO:Al, resistive In—Ga—Zn—O, etc. might be used instead of i-ZnO. TheiZnO layer makes the TFPV device less sensitive to lateralnon-uniformities caused by differences in composition or defectconcentration in the absorber and/or buffer layers. The iZnO layer istypically between about 0 nm and 150 nm in thickness. The iZnO layer istypically formed using a reactive PVD (sputtering) technique or CVDtechnique.

A back contact layer, 512, is formed on intrinsic iZnO layer, 510. Anexample of a suitable back contact layer material is a thin n-type TCOfollowed by Ni and/or Al. The back contact layer may be formed by anynumber of deposition technologies. Examples of suitable depositiontechnologies comprise PVD (sputtering), evaporation, chemical vapordeposition (CVD), atomic layer deposition (ALD), plating, etc. Thethickness of the back contact layer is typically between about 0.3 μmand about 1.0 μm. The back contact layer has a number of requirementssuch as high conductivity, good ohmic contact to the absorber layer,ease of bonding to tabs for external connectivity, ease of scribing orother removal, good thermo-mechanical stability, and chemical resistanceduring subsequent processing, among others. Other types of TFPV devicesuse different materials for the back contact. As an example, Cu-basedmaterials such as Cu/Au, Cu/graphite, Cu/Mo, Cu:ZnTe/Mo, etc. aretypically used for CdTe TFPV devices and TCO materials such as ZnO, ITO,SnO₂:F, etc. are typically used for a-Si TFPV devices.

The film stack described above is just one example of a film stack thatcan be used for TFPV devices. As an example, another substrate filmstack (i.e. similar configuration as FIG. 3) might be:substrate/AZO/Mo/MoSe₂/CIGS/CdS/iZnO/AZO with AZO being ZnO:Al. As anexample, another p-superstrate film stack (i.e. similar configuration asFIG. 5) might be:substrate/barrier/ZnO:Al/Mo/MoSe₂/CIGS/CdS/iZnO/ZnO:Al/Al. The detailedfilm stack configuration is not meant to be limiting, but simply servesas an example of the implementation of embodiments of the presentinvention.

The formation of the absorber layer is typically a multi-step process.One way of grading CIGS materials is by a 2-step approach as illustratedin FIG. 6. In step 602, “metal precursor” films are deposited. ForCIGS-like absorbers, the metal precursor films comprise Group IB andGroup-IIIA metals. In the case of CIGS absorbers, the metal precursorfilms comprise Cu, In, and Ga, with/without a Na source. This metal filmneeds to be converted to one or more chalcogenide compound(s) to formthe absorber layer. The metal precursor film is converted to one or morechalcogenide compound(s) by heating the film in the presence of a sourceof one or more Group-VIA elements as indicated in step 606. Optionally,the chalcogenide film can be annealed as indicated in step 608.

For CIGS-type absorbers, a variation of the 2-step process comprisesdepositing a second thin Group-IIIA-containing film or Group-IIIAchalcogenide material (e.g. Ga—Se, or Al—Se) on top of the metalprecursor film as illustrated in step 604. The Group-IIIA metal is boundin the chalcogenide, its diffusion (e.g. Ga, or Al) toward the back ofthe absorber layer is retarded, yielding a higher concentration of theGroup-IIIA metal at the front of the absorber layer. This results in adouble-graded composition of the Group-IIIA metal and a double-gradedbandgap.

Generally, the 2-step method may comprise more than two steps whenvarious wet chemical and/or conversion methods and/or wet or dry surfacetreatments (e.g. for densification or contaminant removal) and/ordeposition steps (e.g. for a separate chalcogen layer as discussedpreviously) are used to form the metal precursor film. As discussedabove, the metal precursor film may be a single layer or may be formedfrom multiple layers, it may be dense or porous.

The highest efficiencies for 2-step CIG(S)Se have been achieved byconverting PVD (sputtered) Cu(In,Ga) into CIG(S)Se by a chalcogenizationprocess where the Cu(In,Ga) film is both selenized and sulfurized,meaning the final absorber (CIGSSe) contains both selenium and sulfur.Unfortunately, CIG(S)Se formed using a 2-step process has not yetachieved >20% efficiency, and lags behind the laboratory champion ofCIGSe. This is mainly due to the fact that it is challenging to controlboth bandgap grading and maintain a high minority carrier lifetime whensulfur is introduced.

Unfortunately, the traditional 2-step approach based on Cu(In,Ga)followed by selenization (without introducing sulfur) has so far onlyresulted in flat bandgap profiles, or single-graded CIGSe, resulting inefficiencies<16.0%.

It should be noted that the above cited efficiencies are laboratorychampion efficiencies for ˜0.5 cm² solar cells, not to be confused withcommercially available, average, solar panel efficiencies which aretypically 5-6% lower than laboratory champions, due to a combination ofnon-uniformity within solar cells, mismatch between series-connectedcells, absorption losses in thick TCO layers, encapsulant, and glass,scribe and edge losses, and additional series resistance, all inaddition to running a different process in the factory compared to thelaboratory.

One of the main challenges for 2-step selenization is to control thephase separation in the Cu-poor film. High efficiency CIG(S)Se requiresa Cu-poor (p-type) CIGSe film. Cu-poor Cu(In,Ga) metal films prior tochalcogenization are multi-phasic (2 or more separate phases present inthe film), and as such, are hard to deposit in a homogeneous fashionthat provides a conformal, smooth, uniform Cu(In,Ga) film, especially,due to the fact that indium-rich phases have the tendency to agglomeratedue to poor wetting of underlying surfaces. Laterally uniform Cu(In,Ga)and Cu(In,Ga)Se₂ films are needed to avoid the formation of weak diodesthat reduce the overall solar cell efficiency.

The agglomeration of indium is typically minimized by reducing thedynamic deposition rate, and/or controlling the substrate temperatureduring PVD, and/or introducing a multi-layer stack of alternating layersof In-rich and Cu-rich layers, all resulting in additional CapitalExpenditure (CapEx). Other approaches try to avoid the phase separationby depositing a chalcogenide precursor film by PVD from binary, ormultinary chalcogenide targets which results in a CapEx investmenttypically >3× higher than for PVD-CIG due to the deposition of a film˜3× thicker with a lower dynamic deposition rate. In addition, directmaterial costs for the chalcogenide targets are higher than for themetallic targets.

A second challenge for 2-step selenization is to control bandgap gradingin depth in the final CIGSe film by Ga/(In+Ga) compositional grading.Ga-rich phases selenize slower than Cu and In, and therefore, most ofthe Ga collects at the back of the CIGSe film resulting in asingle-graded CIGSe film. One way to avoid this Ga migration andmaintain a flat Ga distribution is to extend the selenization time (>30min), and go to high temperatures (550-600 C), not compatible with alllow-temperature, low-cost substrates. Furthermore, this has not resultedin any double-graded CIGSe (>20%).

A third challenge for 2-step selenization is to prevent adhesion failureof the CIGSe film due to stress resulting from the expansion fromCu(In,Ga) to CIGSe at elevated temperature. The expansion from the metalfilm to the chalcogenide film can be 2.5-3.0× in volume. Additionally,the overall stack of layers may have very different coefficients ofthermal expansion, thickness, and Young's modulus.

A second way of grading CIGS materials is by a 4-step approach asillustrated in FIG. 7. In step 702, “metal precursor” films aredeposited. For CIGS-like absorbers, the metal precursor films compriseGroup IB and Group-IIIA metals. In the case of CIGS absorbers, themetals comprise Cu, In, and Ga, with/without a Na source. This metalprecursor film needs to be converted to a chalcogenide to form theabsorber layer. The metal precursor film is converted (partially orfully) to a chalcogenide by heating the film in the presence of a sourceof one or more Group-VIA elements as indicated in step 704. As usedherein, it will be understood that “partially converted” will beunderstood to mean that at least a portion of the metal precursor filmis converted to a chalcogenide through exposure to a chalcogen atelevated temperature. In step 706, a layer rich in a bandgap-increasingmetal (relative to the metal precursor film deposited in step 702) isformed on the surface of the partially or fully chalcogenized precursorfilm. For example, if the metal precursor film deposited in step 702 isa Cu—In—Ga material, then at least one of Ga/(Ga+In) or Ag/(Ag+Cu) isgreater in the layer deposited in step 706 than in the metal precursorfilm deposited in step 702. In step 706, the layer rich in abandgap-increasing metal may be a metal, a metal alloy, or a metalchalcogenide material (e.g. metal oxide, metal sulfide, metal selenide,metal telluride, etc.). In step 708, the entire precursor stack to formthe final absorber is converted using a chalcogenization process. Thechalcogenization process may include an additional anneal step at theend to improve the device performance as illustrated in step 710.Details of a chalcogenization process including an additional annealstep are described in U.S. patent application Ser. No. 13/283,225,entitled “Method of Fabricating CIGS by Selenization at HighTemperatures”, filed on Oct. 27, 2011, which is herein incorporated byreference.

Generally, the 4-step method may comprise more than 4 steps when variouswet chemical and/or conversion methods and/or wet or dry surfacetreatments (e.g. for densification or contaminant removal) and/ordeposition steps are used to form the metal precursor film and/or themetal rich layer. As discussed above, the metal precursor film and/orthe metal rich layer may each be a single layer or may each be formedfrom multiple layers, it may be dense or porous.

In each of the multi-step methods described herein, the performance ofthe absorber layer can be improved by incorporating a small amount (i.e.about 0.1 atomic %) of Na prior, during, or after the growth of theabsorber layer. The incorporation of Na results in improved filmmorphology, higher conductivity, and beneficial changes in the defectdistribution within the absorber material. The Na may be introduced in anumber of ways. The Na may diffuse out of the glass substrate, out of alayer disposed between the glass substrate and the back contact (e.g. aNa containing sol-gel layer formed under the back contact), or out ofthe back contact (e.g. molybdenum doped with a Na salt). The Na may beintroduced from a separate Na containing layer formed on top of the backcontact. The Na may be introduced by incorporating a Na source in theCu(In, Ga) precursor film. Examples of suitable Na sources compriseNa₂Se, Na₂O₂, NaF, Na₂S, etc. The Na may be introduced from a separateNa containing layer formed on top of the Cu(In, Ga) precursor film. TheNa may be introduced from a separate Na containing layer formed on topof the partially or completely chalcogenized CIGS film. The Na may beintroduced by incorporating a Na source in the Ga-rich film. The Na maybe introduced from a separate Na containing layer formed on top of theGa-rich film. The Na may be introduced by incorporating a Na sourceduring the selenization step. The Na may be introduced after the finalselenization step, followed by a heat treatment. The Na may beintroduced by combining any of these methods as required to improve theperformance of the absorber layer. It should be noted that similar GroupIA, and/or Group IIA elements like K, and Ca might be used instead ofsodium.

In each of the multi-step methods described above and the examples to bedisclosed below, a metal precursor film(s) is deposited. Typically, theprecursor material will deviate in shape, size, composition,homogeneity, crystallinity, or some combination of these parameters fromthe absorber material that is ultimately formed as a result of themethod. As mentioned previously, the metal precursor film(s) cancomprise multiple layers. These layers may be deposited by the same orby different deposition techniques. These layers can be porous, ordense.

The metal precursor film(s) can be deposited using a number oftechniques. Examples comprise dry deposition techniques such as batch orin-line (co)evaporation, batch or in-line PVD (sputtering), ALD, CVD,Plasma enhanced CVD (PECVD), Plasma enhanced ALD (PEALD), atmosphericpressure CVD (APCVD), ultra-fast atmospheric ALD, etc.

The efficiency of a TFPV device depends on the bandgap of the absorbermaterial. The goal is to have the bandgap tuned to the energy range ofthe photons incident on the device. The theoretical upper limit for asingle p-n junction solar cell has been calculated to be about 33 to34%. The peak in the efficiency occurs for values of the bandgap betweenabout 1.0 eV and about 1.5 eV, and more specifically between about 1.3eV and about 1.5 eV. The bandgap for CIGSe films varies smoothly fromCISe=1.00 (i.e. Ga/(Ga+In)=0.0) to CGSe=1.68 (i.e. Ga/(Ga+In)=1.0). Theregion of interest is from Ga/(Ga+In)=0.4 (˜1.23 eV) to Ga/(Ga+In)=0.7(˜1.45 eV).

In high volume manufacturing, the Cu—In—Ga precursor film is typicallydeposited using a PVD (sputtering) process. The deposition system may bea batch system or an in-line system, but the in-line system is preferreddue to higher throughput and lower cost of ownership. The in-line systemmay be continuous (i.e. the substrates move continuously through thesystem) or the in-line system may use a “stop and soak” process whereinthe substrates are transported to a process station where they stopuntil the process is completed. In-line systems typically include anumber of process stations to allow different compositions to bedeposited or to break a long deposition cycle into smaller, balanced,deposition cycles to increase the overall equipment efficiency of thesystem. At each process station, the substrate may be subjected to smalltranslational oscillations to improve the uniformity of the deposition.This oscillation is not considered part of the transport of thesubstrate.

FIG. 8 illustrates an exemplary in-line deposition (e.g. sputtering)system according to some embodiments of the present invention. FIG. 8illustrates a system with three deposition stations, but those skilledin the art will understand that any number of deposition stations can besupplied in the system. For example, the three deposition stationsillustrated in FIG. 8 can be repeated and provide systems with 6, 9, 12,etc. targets, limited only by the desired layer deposition sequence andthe throughput of the system. A transport mechanism 820, such as aconveyor belt or a plurality of rollers, can transfer substrate 840between different deposition stations. For example, the substrate can bepositioned at station #1, comprising a target assembly 860A, thentransferred to station #2, comprising target assembly 860B, and thentransferred to station #3, comprising target assembly 860C. Station #1can be configured to deposit a Cu—In—Ga precursor film. Station #2 canbe configured to deposit an additional Cu—In—Ga precursor film with thesame or different composition. Station #3 can be configured to depositan additional Cu—In—Ga precursor film with the same or differentcomposition.

As noted in the Background section, high-efficiency absorbers areCu-poor (Cu/(In+Ga)<1), and Ga-poor (Ga/(In+Ga)<0.4). However, Cu(In,Ga)metal films with these preferred atomic ratios are multi-phasic and tendto separate into discrete phases when deposited, especially when exposedto processing temperatures above about 155° C. Embodiments of theinstant invention provide layer compositions existing as stable phases.The use of stable phases obviates some of the difficulties in absorberlayer manufacture involving agglomeration of In and phase separation,e.g., excessive changes in grain size and crystallinity, resulting in achange in roughness of the film above 5-10 nm. In some embodiments,targets are used wherein the target material exists as a stable phase.When sputtered onto a substrate or other layers, this material exists inthe same stable phase as its original composition.

Ag can be substituted for part of the Cu in CIGS absorbers. There aremultiple reasons why such a substitution can be beneficial. Ag canincrease the bandgap. The increase in bandgap can be useful in itself,and the difference in bandgap compared to Cu can be exploited to gradethe bandgap across the thickness of the absorber. Such graded bandgaplayers can provide improved conversion of photon energy to electronenergy by reducing recombination. Ag can also lower the structuraldefects in the absorber due to the lower melting temperature of AgInSe₂and AgGaSe₂ compared to CuInSe₂ and CuGaSe₂. For example, AgInSe₂(MP=780° C.) has lower melting temperature compared to CuInSe₂ (MP=986°C.); while AgGaSe_(2 (MP=)850° C.) has lower melting temperaturecompared to CuGaSe₂ (MP=1094° C.)).

However, as discussed in the Background section above, sputtering frompure Ag targets can result in poor material utilization and lowdeposition rates. The deposition rate when using an elemental Ag targetis diminished in comparison with deposition rates when using Ag-metalalloy targets. In addition, sputtered elemental Ag is known toagglomerate, resulting in phase inhomogeneity (roughness). Use of Ag—Inalloys can provide improved deposition rates at higher conformality andprecursor layer quality.

A phase diagram for Ag—In alloys is shown in FIG. 9 (reproduced fromFIG. 1 of Lee et al., “High temperature silver-indium jointsmanufactured at low temperature” Thin Solid Films, 366, (1-2), 196-201,2000, incorporated herein by reference). When a layer is formed from atarget comprising less than 21% In in Ag, the layer is a single stablephase even when temperature is increased. The grain size and surfaceroughness likewise remain substantially stable, and diffusion betweenlayers is minimized, depending on the phase behavior of the aggregate ofthe elements in the adjacent layers. It can be seen that Ag—In forms asolid solution containing up to 21 wt % In over a broad temperaturerange from 0 to 700° C. Homogeneous targets containing up to 21 wt % Inin Ag can therefore be readily made by low-cost methods such as casting.

Similarly, a layer can be formed of a Ag—In composition of approximately26-35 wt. % In, illustrated in the phase diagram shown in FIG. 9(multiple regions). These compositions include the intermetalliccompound Ag₂In, and exist over a broad temperature range up to at least600° C. In addition, a layer can be formed of the intermetallic compoundAgIn₂ (up to 68.1 wt. % In). Above 166° C., it is known that AgIn₂decomposes into liquid In with Ag₂In grains. In addition, a layer can beformed of the Ag₃In, which is stable up to 187° C.

In some embodiments, Ag—In targets are used to form one or more of thethermodynamically stable layers in the precursor film. As will be shownby example below, the number of targets required to make Ag-CIG layerscan be reduced. The deposition rate of Ag using Ag—In alloy targets canbe higher than for pure Ag targets. Both Ag and In tend to agglomerateif deposited separately; co-deposition tends to result in smoother alloyfilms leading to better homogeneity in the finished Ag-CIGS layer.

Grading can also be achieved by selective substitution of Ag for Cu andS for Se in a graded fashion. The addition of Ag can also lowerstructural defects in the absorber due to the lower melting temperatureof AgInSe₂, AgGaSe₂, AgInS₂, AgGaS₂ compared to CISe, CGSe, CIS(sulfur), CGS (sulfur), CIGSe, and CIGSSe. In order to introduce Ag intothe layer, Ag sputtering targets have been used, for example, incombination with sequential sputtering from In, Cu, and Cu_(x)Ga_(y)targets. However, the use of elemental Ag targets, like the use of Intargets, can result in agglomerated and inhomogeneous deposition ofsputtered material. In addition, the use of Ag targets to form absorberlayers results in low deposition rates and poor material utilizationrate for the relatively expensive Ag if target and tool design is notoptimized.

In some embodiments, the phase separation can be avoided by depositingalternating layers of a Cu-rich and Ga-rich Cu—In—Ga layer from aCu—In—Ga target and a pure In layer from an In target. Indium readilywets a CIG layer, and the CIG layer is thermodynamically stable forCu/(In+Ga)>2 and Ga/(Ga+In)>0.5. A CIG layer with these atomicproportions does not exhibit agglomeration or segregation. Preferably,Ga/(Ga+In)>0.6. Most of the In is deposited in the pure In layer. Thestability of the CIG layer with low In content is apparent in theternary phase diagram shown in FIG. 10 (from FIG. 4 of Purwins et al.,“Phase relations in the ternary Cu—Ga—In system,” Thin Solid Films 515,5895-98, 2007, incorporated herein by reference).

In some embodiments, elemental metal targets consisting essentially ofIn or Cu can be used to form precursor layers comprising elementalmetals or alloys. The stability of the stack of layers is dictated bythe phase diagram of the elements involved, not the stability of onelayer by itself. For example, a stack of precursor layers having thecomposition Cu₂(In_(0.25),Ga_(0.75)) and In is thermodynamically stableup to several hundred degrees. Use of stable precursor layers results inminimal interdiffusion between layers and minimal morphological changeslaterally.

Cu—Ga targets can be used wherein the Cu and Ga composition is such thatthe material exists in a stable phase. A ternary phase diagram is shownin FIG. 10, illustrating the phases existing in ternary mixtures of Cu,Ga and In. Binary targets comprising Cu and Ga can include the followingcompositions: CuGa₂, Cu₁₆Ga₁₁, Cu₆₀Ga₄₀, Cu₉Ga₅, Cu₆₅Ga₃₅, Cu₆₈Ga₃₂,Cu₉Ga₄ Cu₇₅Ga₂₅, Cu₈₅Ga₁₅. Ternary target compositions can be used,including Cu₉(Ga_(0.75)In_(0.25))₄ Cu₁₁(Ga_(0.35)In_(0.65))₄Cu₁₁(Ga_(0.45)In_(0.55))₄ Cu₁₁(Ga_(0.55)In_(0.45))₄Cu_(0.85)(Ga_(0.25)In_(0.75)) Cu_(0.85)(Ga_(0.35)In_(0.65))Cu_(0.85)(Ga_(0.45)In_(0.55)) Cu_(0.83)(Ga_(0.3)In_(0.7)).

As an exemplary composition, if x is the number of In atoms in the CIGlayer, then the number of Ga atoms can be 1.5x (giving Ga/(Ga+In)=0.6).The number of Cu atoms in the CIG layer can be at least 5x (givingCu/(Ga+In)>2). The In layer can contain 3x In atoms. The resulting layerpairs contain the desired Cu-poor and Ga-poor compositions:(Cu/(In+Ga)>0.9; Ga/(In+Ga)=0.27. Subsequent selenization and/orsulfurization of the CIG film can be performed without risk of phaseseparation in an early stage of the heating process. In terms of atomicpercentages, the exemplary CIG target and layer comprise at least 67atomic percent Cu, with 20 atomic percent gallium and 13 atomic percentindium (the Ga and In being reduced proportionally as the Cu isincreased). The total thickness of deposited In layers is adjusted tocontain three times the amount of In in the CIG layer. Other similarcompositions can be made that meet the thermodynamic stabilitylimitations for the CIG target with the In layer thickness adjusted toachieve the desired net CIG composition ratios in the finished absorber.

In some embodiments, phase separation can be avoided by using a set ofthree targets: Cu—Ga, Cu, and In₂Se₃ (and/or In₂S₃). These targets canbe used sequentially to deposit layers which are individually stable toabove 400° C. and can be deposited conformally with little or noagglomeration. Multiple sets of the three layers can be deposited toachieve a desired total thickness and to allow for grading of thecomposition across the total thickness. In some embodiments, the In₂Se₃and/or In₂S₃ layers can be formed from an In metal target by reactivesputtering in an atmosphere comprising H₂S and/or H₂Se and/or Se and/orS. The amount of S or Se incorporated into the layer from these threetargets is generally not sufficient to fully selenize/sulfurize themetals, and a further selenization/sulfurization step can be implementedin a batch or inline furnace to complete the formation of the desiredabsorber composition. Because the three precursor layers areindividually stable, no agglomeration occurs during deposition and theinitial stages of the conversion step. Ga migration can also be reducedcompared to that which occurs when individual metal targets are used todeposit the metals, and the grading of Ga content across the finishedCIGS layer can be better controlled.

Cu—Ga targets are used wherein the Cu—Ga target is formed using hot(isostatic pressure) techniques to form the targets from (atomized)powders. This technique allows a broad range of Ga concentrations to berealized. Additionally, other elements may be included within the targetsuch as Ag and In, and optionally, a source of Na, or similar elementlike K, Mg, or Ca. The targets will be largely metallic and thesputtering yields of these components are similar in the typicalprocessing ranges used to deposit Cu—In—Ga precursor films. Therefore,it is not a requirement that the targets be manufactured as single phaseor with a composition that corresponds to an equilibrium compound.

In some embodiments, the Ga content is greater than about 25 atomic %.In some embodiments, the target includes a Cu concentration of about 60atomic % and a Ga concentration of about 40 atomic % (i.e. Cu₆₀Ga₄₀). Insome embodiments, the target includes a Cu concentration of about 33.3atomic % and a Ga concentration of about 66.6 atomic % (i.e. Cu₁Ga₂). Insome embodiments, the target includes a Cu—In_(y)—Ga_(x) materialwherein (x+y) is between about 25 atomic % and about 66 atomic %. Insome embodiments, the target includes a Cu_(w)Ag_(z)—In_(y)Ga_(x)material wherein (x+y) is between about 25 atomic % and about 66 atomic%. In some embodiments, the targets comprise Ag and In containing lessthan 21% In by weight. In some embodiments, the target comprises Cu andGa comprising less than 45% Ga by weight. In some embodiments, thetarget comprises Cu(In,Ga), wherein the Cu(In,Ga) target has an atomratio of Cu to (In+Ga) greater than 2 and an atom ratio of Ga to (Ga+In)greater than 0.5. In some embodiments, the target comprisesCu₂(In_(x)Ga_(1−x)), x=0.25. In some embodiments, the target compriseselemental In and is used in a chalcogenizing atmosphere. In someembodiments, the target is In₂Se₃, In₂S₃, or In₂(S,Se)₃ or mixturesthereof.

Table 1 lists a number of potential target compositions that can be usedto deposit precursor layers in accordance with embodiments of theinvention. A wide variety of target compositions can be utilized so longas the precursor layers deposited exhibit thermodynamically stable phasebehavior. A plurality of targets can be used for simultaneous depositionin order to form a desired composition in a particular precursor layer.Those skilled in the art will understand that the potential targetcompositions listed in Table 1 are exemplary and that other potentialtarget compositions are possible. One of skill in the art will recognizethat the listed targets do not necessarily match the composition oflayers to be formed in some embodiments, and it may be necessary to usea combination of two or more targets from Table 1 to achieve the desiredlayer composition. The present invention is not limited to the specificexamples listed in Table 1; any combination of targets can be used thatenable the deposition of layers having the desired composition.

TABLE 1 Target compositions for Cu—In—Ga precursor layers Formula forcomposition Cu Ga In At. % Cu At. % Ga At. % In In 0.00 0.00 1.00 0.000.00 100.00 CuIn₂ 1.00 0.00 2.00 33.33 0.00 66.67 Cu₁₁In₉ 11.00 0.009.00 55.00 0.00 45.00 Cu₉In₅ 9.00 0.00 5.00 64.29 0.00 35.71 Cu₉In₄ 9.000.00 4.00 69.23 0.00 30.77 CuGa₂ 1.00 2.00 0.00 33.33 66.67 0.00Cu₁₆Ga₁₁ 16.00 11.00 0.00 59.26 40.74 0.00 Cu₆₀Ga₄₀ 60.00 40.00 0.0060.00 40.00 0.00 Cu₉Ga₅ 9.00 5.00 0.00 64.29 35.71 0.00 Cu₆₅Ga₃₅ 65.0035.00 0.00 65.00 35.00 0.00 Cu₆₈Ga₃₂ 68.00 32.00 0.00 68.00 32.00 0.00Cu₉Ga₄ 9.00 4.00 0.00 69.23 30.77 0.00 Cu₇₅Ga₂₅ 75.00 25.00 0.00 75.0025.00 0.00 Cu₈₅Ga₁₅ 85.00 15.00 0.00 85.00 15.00 0.00Cu₉(Ga_(0.75)In_(0.25))₄ 9.00 3.00 1.00 69.23 23.08 7.69Cu₁₁(Ga_(0.35)In_(0.65))₉ 11.00 1.4 2.6 55.00 15.75 29.25Cu₁₁(Ga_(0.45)In_(0.55))₉ 11.00 1.8 2.2 55.00 20.25 24.75Cu₁₁(Ga_(0.55)In_(0.45))₉ 11.00 2.2 1.8 55.00 24.75 20.25Cu_(0.85)(Ga_(0.25)In_(0.75)) 0.85 0.25 0.75 45.95 13.51 40.54Cu_(0.85)(Ga_(0.35)In_(0.65)) 0.85 0.35 0.65 45.95 18.92 35.14Cu_(0.85)(Ga_(0.45)In_(0.55)) 0.85 0.45 0.55 45.95 24.32 29.73Cu_(0.83)(Ga_(0.3)In_(0.7)) 0.83 0.30 0.70 45.36 16.39 38.25

In addition, chalcogenide-based compositions can also be included aspossible target compositions for depositing chalcogenized layers alongwith the precursor layers. For example, as discussed above, In₂S₃ andIn₂Se₃ targets can be utilized to deposit thermodynamically stablelayers comprising In₂S₃ and In₂Se₃.

For deposition of Ag—In precursor layers, exemplary targets comprise Agand In having the same compositions as for the precursor layers,i.e., 1) containing less than 21% In by weight, 2) 26-35 wt % In, or 3)AgIn₂. These compositions are illustrated in Table 2. The compositionsare shown as wt % only. Due to the similar atomic weight of Ag and In,the wt % is approximately the same as the atomic %. Targets comprisingAg and In in other proportions can be used if the deposition from eachtarget is controlled so as to deposit precursor layers of the desiredcompositions, as described above. Targets can be manufactured usingconventional methods such as casting or pressing of (atomized) powders.Use of higher In-containing Ag—In alloys allows for less deposition ofelemental indium, and can provide greater stability for the precursorlayer stack as it is heated.

TABLE 2 Target compositions for Ag—In precursor layers Formula forcomposition Ag In At. % Ag At. % In Ag_(>0.79)In_(<0.21) >0.79<0.21 >79.0 <21.0 Ag_(0.65-0.74)In_(0.26-.35) 0.65-0.74 0.26-0.3565.0-74.0 26.0-35.0 AgIn₂ 1 2.00 33.33 66.67

Table 3 illustrates examples of deposition configurations that may beused to produce Ag,Cu—In—Ga precursor films. Table 3 illustrates fourexamples of deposition configurations. Those skilled in the art willunderstand that other configurations are possible and would fall withinthe scope of the present invention.

TABLE 3 Deposition configurations ID Deposition # 1 Deposition # 2Deposition # 3 1 Ag—In with less Cu—Ga with Ga less In than 21 wt % Inthan 45 wt % Ag—In with 26-35 Cu—Ga with Ga less In (optional) wt % Inthan 45 wt % AgIn₂ Cu—Ga with Ga less In (optional) than 45 wt % 2 Cu(In, Ga) In — 3 Cu Cu—Ga In₂Se₃ and/or In₂S₃ (inert atmosphere)

In the configuration with ID=1, the first deposition is used to form alayer that is composed of less than 21 wt % In in Ag. This layer mayalso include other elements at lower concentrations so long as thematerial forms a stable phase. This layer may be formed using any one ofPVD (sputtering), evaporation, plating, or other deposition methodsdiscussed previously. The second deposition is used to form a layer thatis primarily composed of Cu and Ga, with the Ga composition less than 45wt %. This layer may also include other elements at lowerconcentrations. This layer may be formed using any one of PVD(sputtering), evaporation, plating, or other deposition methodsdiscussed previously. Those skilled in the art will understand that theabsorber material formed after the conversion to a chalcogenide andafter the interdiffusion will advantageously be Cu poor. The thirddeposition is used to form a layer that is primarily composed ofelemental indium. This layer may also include other elements at lowerconcentrations so long as the material forms a stable phase. This layermay be formed using any one of PVD (sputtering), evaporation, plating,or other deposition methods discussed previously. In some embodiments,the third deposition forms a layer that is primarily composed of In₂Se₃and/or In₂S₃, for example, by performing the deposition using an Intarget in the presence of a chalcogen, or by using a target comprisingIn₂Se₃ and/or In₂S₃. Those skilled in the art will understand that theabsorber material formed after the conversion to a chalcogenide andafter the interdiffusion will advantageously be Cu poor.

In some embodiments, the first deposition is used to form a layer thatis composed of 26-35 wt % In in Ag. This layer may also include otherelements at lower concentrations so long as the material forms a stablephase. This layer may be formed using any one of PVD (sputtering),evaporation, plating, or other deposition methods discussed previously.The second deposition is used to form a layer that is primarily composedof Cu and Ga, with the Ga composition less than 45 wt %. This layer mayalso include other elements at lower concentrations. This layer may beformed using any one of PVD (sputtering), evaporation, plating, or otherdeposition methods discussed previously. Those skilled in the art willunderstand that the absorber material formed after the conversion to achalcogenide and after the interdiffusion will advantageously be Cupoor. The third deposition is used to form a layer that is primarilycomposed of elemental indium. This layer may also include other elementsat lower concentrations so long as the material forms a stable phase.This layer may be formed using any one of PVD (sputtering), evaporation,plating, or other deposition methods discussed previously. Those skilledin the art will understand that the absorber material formed after theconversion to a chalcogenide and after the interdiffusion willadvantageously be Cu poor.

In some embodiments, the first deposition is used to form a layer thatis composed of AgIn₂. This layer may also include other elements atlower concentrations so long as the material forms a stable phase. Thislayer may be formed using any one of PVD (sputtering), evaporation,plating, or other deposition methods discussed previously. The seconddeposition is used to form a layer that is primarily composed of Cu andGa, with the Ga composition less than 45 wt %. This layer may alsoinclude other elements at lower concentrations. This layer may be formedusing any one of PVD (sputtering), evaporation, plating, or otherdeposition methods discussed previously. Those skilled in the art willunderstand that the absorber material formed after the conversion to achalcogenide and after the interdiffusion will advantageously be Cupoor. The third deposition is used to form a layer that is primarilycomposed of elemental indium. This layer may also include other elementsat lower concentrations so long as the material forms a stable phase.This layer may be formed using any one of PVD (sputtering), evaporation,plating, or other deposition methods discussed previously. Those skilledin the art will understand that the absorber material formed after theconversion to a chalcogenide and after the interdiffusion willadvantageously be Cu poor.

In the configuration with ID=2, the first deposition is used to form alayer that is composed of Cu(In,Ga) having a Cu-rich composition. Thatis, the Cu/(In+Ga) ratio is greater than about 2, and the Ga/(In+Ga)ratio is above 0.5. This layer may also include other elements at lowerconcentrations so long as the material forms a stable phase. This layermay be formed using any one of PVD (sputtering), evaporation, plating,or other deposition methods discussed previously. The second depositionis used to form a layer that is primarily composed of elemental In. Thislayer may also include other elements at lower concentrations so long asthe material forms a stable phase. This layer may be formed using anyone of PVD (sputtering), evaporation, plating, or other depositionmethods discussed previously. In some embodiments, the second depositionforms a layer that is primarily composed of In₂Se₃ and/or In₂S₃, forexample, by performing the deposition using an In target in the presenceof a chalcogen, or by using a target comprising In₂Se₃ and/or In₂S₃.Those skilled in the art will understand that the absorber materialformed after the conversion to a chalcogenide and after theinterdiffusion will advantageously be Cu poor.

In the configuration with ID=3, the first deposition is used to form alayer that is primarily composed of elemental Cu. This layer may alsoinclude other elements at lower concentrations so long as the materialforms a stable phase. This layer may be formed using any one of PVD(sputtering), evaporation, plating, or other deposition methodsdiscussed previously. The second deposition is used to form a layer thatis primarily composed of Cu—Ga. This layer may also include otherelements at lower concentrations so long as the material forms a stablephase. This layer may be formed using PVD (sputtering) using a high Gacontent target that has a Ga composition (Ga/(Ga+In)) in the range ofabout 0.25 to about 0.66 as discussed previously. The third depositionis used to form a layer that is primarily composed of In₂Se₃ and/orIn₂S₃. This layer may also include other elements at lowerconcentrations so long as the material forms a stable phase. This layermay be formed using any one of PVD (sputtering), evaporation, plating,or other deposition methods discussed previously using an inertatmosphere. The third deposition can also be performed by PVD using anIn target in the presence of a chalcogen, or by using a targetcomprising In₂Se₃ and/or In₂S₃. Those skilled in the art will understandthat the absorber material formed after the conversion to a chalcogenideand after the interdiffusion will advantageously be Cu poor.

In the configuration with ID=4, the first deposition is used to form alayer that is primarily composed of elemental Cu. This layer may alsoinclude other elements at lower concentrations so long as the materialforms a stable phase. This layer may be formed using any one of PVD(sputtering), evaporation, plating, or other deposition methodsdiscussed previously. The second deposition is used to form a layer thatis primarily composed of Cu—Ga. This layer may also include otherelements at lower concentrations so long as the material forms a stablephase. This layer may be formed using PVD (sputtering) using a high Gacontent target that has a Ga composition (Ga/(Ga+In)) in the range ofabout 0.25 to about 0.66 as discussed previously. The third depositionis used to form a layer that is primarily composed of In₂Se₃ and/orIn₂S₃. This layer may also include other elements at lowerconcentrations so long as the material forms a stable phase. The thirddeposition can also be performed by PVD using an In target in thepresence of a chalcogen, or by using a target comprising In₂Se₃ and/orIn₂S₃. Those skilled in the art will understand that the absorbermaterial formed after the conversion to a chalcogenide and after theinterdiffusion will advantageously be Cu poor.

Those skilled in the art will understand that in each of the exampleslisted in Table 3, a portion of the Cu in any of the Cu-containinglayers may be substituted with Ag so long as the material forms a stablephase. In some embodiments, a system similar to that illustrated in FIG.8 may have two or more deposition stations, typically to deposit a firstmetal precursor film and a second layer rich in a bandgap-increasingmetal.

In some embodiments, Na (or other Group-IA or Group-IIA elements) can beincorporated into the final absorber material. The Na (or othermaterials) can be incorporated into one or more of the metal targets orcan be deposited as a separate layer. The Na (or other materials) can bedeposited at the beginning of the deposition of the precursor layer(s),as an intermediate layer, or at the end of the deposition of theprecursor layer(s).

In some embodiments, a capping layer (e.g. Se) can be deposited toprotect the precursor layer(s) from the ambient environment prior to thechalcogenization step.

It is desirable to optimize the selenization of the metal precursorfilms by increasing the reaction temperature. At elevated temperatures,In agglomeration competes against selenization. Under conditions whereIn agglomeration occurs despite selenization, In particulates segregatefrom the metal precursor film and form separate binary phases. Theresulting films have a spotty and blister-like visual appearance. X-raydiffraction (XRD) spectra on these films show InSe (004) and (006) peaksin addition to the ternary chalcopyrite phases. There is nophoto-luminescence (PL) intensity on these films and device results arepoor. To solve this problem, an intermediate, lower temperatureselenization step at between about 300° C. and about 450° C. can beperformed as described in U.S. patent application Ser. No. 13/595,730,incorporated by reference herein, to partially selenize the precursorfilm before complete selenization at higher temperatures. The additionalstep secures the indium in a selenized state to prevent agglomeration.This eliminates the problem of binary phase formation in the finalselenization step.

Another problem typically encountered during selenization is thedifficulty to control the degree of selenization at higher temperatures.The selenization reaction of CIGSe occurs at temperatures above about350° C. if the Se source is H₂Se. In an exemplary batch furnace, theramp rates are generally limited to about 10° C./min by hardware. Thoseskilled in the art will understand that parameters such as ramp ratesand temperature uniformity within processing equipment depend on thedetails of the equipment and that exemplary values used herein are notlimiting. If the furnace temperature is increased to 600° C.,delamination at the Mo/CIGSe interface is observed due toover-selenization and formation of a thick MoSe₂ layer. In someembodiments of the present invention, a fast gas exchange step isintroduced at the high temperature step to replace H₂Se in the furnacewith an inert gas such as Ar, N₂, etc. to stop further selenization.This resolves the delamination problem due to over-selenization bylimiting the formation of the MoSe₂ layer. Details of the fast gasexchange process are described in U.S. patent application Ser. No.13/283,225 entitled “Method of Fabricating CIGS by Selenization at HighTemperature,” filed on Oct. 27, 2011, herein incorporated by reference.

FIG. 11 illustrates a flow chart that describes methods of someembodiments of the present invention. The purpose of the method is toform a Group I-III-VI semiconductor material suitable for thin filmsolar cells on a substrate. Examples of suitable substrates comprisefloat glass, low-iron glass, borosilicate glass, flexible glass,specialty glass for high temperature processing, stainless steel, carbonsteel, aluminum, copper, titanium, molybdenum, polyimide, plastics,cladded metal foils, etc. Furthermore, the substrates may be processedin many configurations such as single substrate processing, multiplesubstrate batch processing, in-line continuous processing, roll-to-rollprocessing, etc. Those skilled in the art will understand that thesubstrate will have been exposed to several previous processing stepsand will have several layers formed thereon before this step in themanufacture of a solar cell.

In step 1102, a first layer is formed above a surface of the substrate.In some embodiments, the first layer comprises Ag—In containing lessthan 21 wt % In. The first layer may be formed by any one of a varietyof techniques. Examples of suitable techniques include batch or in-line(co)evaporation, batch or in-line PVD (sputtering), ALD, CVD, Plasmaenhanced CVD (PECVD), Plasma enhanced ALD (PEALD), atmospheric pressureCVD (APCVD), ultra-fast atmospheric ALD, plating, printing or sprayingof inks, screen printing, inkjet printing, slot die coating, gravureprinting, or wet chemical depositions.

In step 1104, a second layer is formed above the first layer. In someembodiments, the second layer comprises Cu—Ga, with the Ga compositionless than 45 wt %. Advantageously, the second layer is formed from oneof the Cu—Ga target compositions discussed earlier in reference toTable 1. The second layer may be formed by any one of a variety oftechniques. Examples of suitable techniques include batch or in-line(co)evaporation, batch or in-line PVD (sputtering), ALD, CVD, Plasmaenhanced CVD (PECVD), Plasma enhanced ALD (PEALD), atmospheric pressureCVD (APCVD), ultra-fast atmospheric ALD, plating, printing or sprayingof inks, screen printing, inkjet printing, slot die coating, gravureprinting, or wet chemical depositions.

In step 1106, a third layer is formed above the second layer. In someembodiments, the third layer comprises In and is formed using PVD froman In target. In some embodiments, the third layer comprises In₂Se₃and/or In₂S₃ formed by a batch or in-line PVD (sputtering) techniqueusing reactive sputtering in a selenizing and/or sulfuring atmosphere.In some embodiments, the third layer comprises In₂Se₃ and/or In₂S₃ andis formed by a batch or in-line PVD (sputtering) using an In₂Se₃ and/orIn₂S₃ target in an inert atmosphere. Examples of suitable techniquesinclude batch or in-line (co)evaporation, batch or in-line PVD(sputtering), ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhanced ALD(PEALD), atmospheric pressure CVD (APCVD), ultra-fast atmospheric ALD,plating, printing or spraying of inks, screen printing, inkjet printing,slot die coating, gravure printing, or wet chemical depositions.

The forming steps can be repeated to form an absorber layer of desiredthickness and composition. At this point, the three layers have formed ametal precursor film having stable layers. The finished atomiccomposition ratios can be described as 0.7<(Ag+Cu)/(In+Ga)<1.0 and0<Ga/(In+Ga)<0.4. The metal precursor film is advantageously Cu-poor sothat the absorber material will exhibit p-type behavior.

In step 1108, the metal precursor film is fully converted to achalcogenide material. The converting step may be accomplished using oneof a batch furnace, RTP system, or in-line system. In the case of abatch furnace, the chalcogenization can proceed in accordance with oneor more temperature profiles.

Step 1110 illustrates an optional anneal to allow the interdiffusion ofIn and Ga within the film as discussed previously. This step may beintegrated in the chalcogenization process or may be a separate step.

In some embodiments, a fourth layer that includes Ag is formed above thethird layer before the chalcogenization step (e.g. between steps 1106and 1108—not shown). The addition of Ag provides an additional mechanismfor altering the bandgap grading and improving the performance of thedevice.

In some embodiments, the metal precursor film further includes Na. TheNa may be added during any of the layer formation steps (i.e. steps1102-1106—not shown) or may be added as a separate layer before thechalcogenization step as discussed previously.

In some embodiments, the method illustrated in FIG. 11 excludes thepurposeful addition or exposure of the metal precursor film to a sourceof S. The addition of S complicates the chalcogenization of the metalprecursor film and the formation of the chalcogenide material because ofthe introduction of a second reactant (S) in addition to the existingreactant (Se).

In some additional embodiments, the first layer comprises a greateramount of In in Ag. In some embodiments, the first layer comprises aAg—In composition comprising from 26-35 wt.-% In. In some embodiments,the first layer comprises AgIn₂. The first layer may be formed by anyone of a variety of techniques. Examples of suitable techniques includebatch or in-line (co)evaporation, batch or in-line PVD (sputtering),ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhanced ALD (PEALD),atmospheric pressure CVD (APCVD), ultra-fast atmospheric ALD, plating,printing or spraying of inks, screen printing, inkjet printing, slot diecoating, gravure printing, or wet chemical depositions. FIG. 12illustrates a flow chart that describes methods of some embodiments ofthe present invention. The purpose of the method is to form a GroupI-III-VI semiconductor material suitable for thin film solar cells on asubstrate. Examples of suitable substrates comprise float glass,low-iron glass, borosilicate glass, flexible glass, specialty glass forhigh temperature processing, stainless steel, carbon steel, aluminum,copper, titanium, molybdenum, polyimide, plastics, cladded metal foils,etc. Furthermore, the substrates may be processed in many configurationssuch as single substrate processing, multiple substrate batchprocessing, in-line continuous processing, roll-to-roll processing, etc.Those skilled in the art will understand that the substrate will havebeen exposed to several previous processing steps and will have severallayers formed thereon before this step in the manufacture of a solarcell.

In step 1202, a first layer is formed above a surface of the substrate.In some embodiments, the first layer comprises Cu(In,Ga). That is, theCu/(In+Ga) ratio is greater than about 2, and the Ga/(In+Ga) ratio isabove 0.5. Advantageously, the first layer is formed from one of theCu(In,Ga) target compositions discussed earlier in reference to Table 1.The first layer may be formed by any one of a variety of techniques.Examples of suitable techniques include batch or in-line(co)evaporation, batch or in-line PVD (sputtering), ALD, CVD, Plasmaenhanced CVD (PECVD), Plasma enhanced ALD (PEALD), atmospheric pressureCVD (APCVD), ultra-fast atmospheric ALD, plating, printing or sprayingof inks, screen printing, inkjet printing, slot die coating, gravureprinting, or wet chemical depositions.

In step 1204, a second layer is formed above the first layer. In someembodiments, the second layer comprises In. The second layer may beformed by any one of a variety of techniques. Examples of suitabletechniques include batch or in-line (co)evaporation, batch or in-linePVD (sputtering), ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhancedALD (PEALD), atmospheric pressure CVD (APCVD), ultra-fast atmosphericALD, plating, printing or spraying of inks, screen printing, inkjetprinting, slot die coating, gravure printing, or wet chemicaldepositions.

The forming steps can be repeated to form an absorber layer of desiredthickness and composition. At this point, the two layers have formed ametal precursor film having stable layers. The metal precursor film isadvantageously Cu-poor so that the absorber material will exhibit p-typebehavior.

In step 1208, the metal precursor film is fully converted to achalcogenide material. The converting step may be accomplished using oneof a batch furnace, RTP system, or in-line system. In the case of abatch furnace, a temperature profile can be used to effectchalcogenization process.

Step 1210 illustrates an optional anneal to allow the interdiffusion ofIn and Ga within the film as discussed previously. This step may beintegrated in the chalcogenization process (e.g. step 1208 as discussedpreviously) or may be a separate step.

In some embodiments, a third layer that includes Ag is formed above thesecond layer before the chalcogenization step (e.g. between steps 1204and 1208—not shown). The addition of Ag provides an additional mechanismfor altering the bandgap grading and improving the performance of thedevice.

In some embodiments, the metal precursor film further includes Na. TheNa may be added during any of the layer formation steps (i.e. steps1202-1204—not shown) or may be added as a separate layer before thechalcogenization step as discussed previously.

In some embodiments, the method illustrated in FIG. 12 excludes thepurposeful addition or exposure of the metal precursor film to a sourceof S. As discussed previously, the addition of S complicates thechalcogenization of the metal precursor film and the formation of thechalcogenide material.

FIG. 13 illustrates a flow chart that describes methods of someembodiments of the present invention. The purpose of the method is toform a Group I-III-VI semiconductor material suitable for thin filmsolar cells on a substrate. Examples of suitable substrates comprisefloat glass, low-iron glass, borosilicate glass, flexible glass,specialty glass for high temperature processing, stainless steel, carbonsteel, aluminum, copper, titanium, molybdenum, polyimide, plastics,cladded metal foils, etc. Furthermore, the substrates may be processedin many configurations such as single substrate processing, multiplesubstrate batch processing, in-line continuous processing, roll-to-rollprocessing, etc. Those skilled in the art will understand that thesubstrate will have been exposed to several previous processing stepsand will have several layers formed thereon before this step in themanufacture of a solar cell.

In step 1302, a first layer is formed above a surface of the substrate.In some embodiments, the first layer comprises In₂Se₃ and/or In₂S₃. Thefirst layer may be formed by a batch or in-line PVD (sputtering)technique using reactive sputtering in a selenizing and/or sulfurizingatmosphere. The first layer is formed from an In target.

In step 1304, a second layer is formed above the first layer. In someembodiments, the second layer comprises Cu. The second layer may beformed by any one of a variety of techniques. Examples of suitabletechniques include batch or in-line (co)evaporation, batch or in-linePVD (sputtering), ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhancedALD (PEALD), atmospheric pressure CVD (APCVD), ultra-fast atmosphericALD, plating, printing or spraying of inks, screen printing, inkjetprinting, slot die coating, gravure printing, or wet chemicaldepositions.

In step 1306, a third layer is formed above second first layer. In someembodiments, the third layer comprises Cu—Ga. Advantageously, the firstlayer can be formed from one of the Cu—Ga target compositions discussedearlier in reference to Table 1. The third layer may be formed by anyone of a variety of techniques. Examples of suitable techniques includebatch or in-line (co)evaporation, batch or in-line PVD (sputtering),ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhanced ALD (PEALD),atmospheric pressure CVD (APCVD), ultra-fast atmospheric ALD, plating,printing or spraying of inks, screen printing, inkjet printing, slot diecoating, gravure printing, or wet chemical depositions.

The forming steps can be repeated to form an absorber layer of desireddepth and composition. At this point, the three layers have formed ametal precursor film having stable layers. The metal precursor film isadvantageously Cu-poor so that the absorber material will exhibit p-typebehavior.

In step 1308, the metal precursor film is fully converted to achalcogenide material. The converting step may be accomplished using oneof a batch furnace, RTP system, or in-line system. In the case of abatch furnace, a temperature profile can be used to effect thechalcogenization process.

Step 1310 illustrates an optional anneal to allow the interdiffusion ofIn and Ga within the film as discussed previously. This step may beintegrated in the chalcogenization process (e.g. step 1308 as discussedpreviously) or may be a separate step.

In some embodiments, a fourth layer that includes Ag can be formed abovethe third layer before the chalcogenization step (e.g. between steps1306 and 1308—not shown). The addition of Ag provides an additionalmechanism for altering the bandgap grading and improving the performanceof the device.

In some embodiments, the metal precursor film further includes Na. TheNa may be added during any of the layer formation steps (i.e. steps1302-1306—not shown) or may be added as a separate layer before thechalcogenization step as discussed previously.

In some embodiments, the method illustrated in FIG. 13 excludes thepurposeful addition or exposure of the metal precursor film to a sourceof S. As discussed previously, the addition of S complicates thechalcogenization of the metal precursor film and the formation of thechalcogenide material.

FIG. 14 illustrates a flow chart that describes methods of someembodiments of the present invention. The purpose of the method is toform a Group I-III-VI semiconductor material suitable for thin filmsolar cells on a substrate. Examples of suitable substrates comprisefloat glass, low-iron glass, borosilicate glass, flexible glass,specialty glass for high temperature processing, stainless steel, carbonsteel, aluminum, copper, titanium, molybdenum, polyimide, plastics,cladded metal foils, etc. Furthermore, the substrates may be processedin many configurations such as single substrate processing, multiplesubstrate batch processing, in-line continuous processing, roll-to-rollprocessing, etc. Those skilled in the art will understand that thesubstrate will have been exposed to several previous processing stepsand will have several layers formed thereon before this step in themanufacture of a solar cell.

In step 1402, a first layer is formed above a surface of the substrate.In some embodiments, the first layer comprises In₂Se₃ and/or In₂₅₃. Thefirst layer may be formed by a batch or in-line PVD (sputtering) usingan In₂Se₃ and/or In₂₅₃ target in an inert atmosphere.

In step 1404, a second layer is formed above the first layer. In someembodiments, the second layer comprises Cu. The second layer may beformed by any one of a variety of techniques. Examples of suitabletechniques include batch or in-line (co)evaporation, batch or in-linePVD (sputtering), ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhancedALD (PEALD), atmospheric pressure CVD (APCVD), ultra-fast atmosphericALD, plating, printing or spraying of inks, screen printing, inkjetprinting, slot die coating, gravure printing, or wet chemicaldepositions.

In step 1406, a third layer is formed above the second layer. In someembodiments, the third layer comprises Cu—Ga. Advantageously, the thirdlayer can be formed from one of the Cu—Ga target compositions discussedearlier in reference to Table 1. The third layer may be formed by anyone of a variety of techniques. Examples of suitable techniques includebatch or in-line (co)evaporation, batch or in-line PVD (sputtering),ALD, CVD, Plasma enhanced CVD (PECVD), Plasma enhanced ALD (PEALD),atmospheric pressure CVD (APCVD), ultra-fast atmospheric ALD, plating,printing or spraying of inks, screen printing, inkjet printing, slot diecoating, gravure printing, or wet chemical depositions.

The forming steps can be repeated to form an absorber layer of desiredthickness and composition. At this point, the three layers have formed ametal precursor film having stable layers. The metal precursor film isadvantageously Cu-poor so that the absorber material will exhibit p-typebehavior.

In step 1408, the metal precursor film is fully converted to achalcogenide material. The converting step may be accomplished using oneof a batch furnace, RTP system, or in-line system. In the case of abatch furnace, a temperature profile can be used to effect thechalcogenization process.

Step 1410 illustrates an optional anneal to allow the interdiffusion ofIn and Ga within the film as discussed previously. This step may beintegrated in the chalcogenization process (e.g. step 1308 as discussedpreviously) or may be a separate step.

In some embodiments, a fourth layer that includes Ag can be formed abovethe third layer before the chalcogenization step (e.g. between steps1406 and 1408—not shown). The addition of Ag provides an additionalmechanism for altering the bandgap grading and improving the performanceof the device.

In some embodiments, the metal precursor film further includes Na. TheNa may be added during any of the layer formation steps (i.e. steps1402-1406—not shown) or may be added as a separate layer before thechalcogenization step as discussed previously.

In some embodiments, the method illustrated in FIG. 14 excludes thepurposeful addition or exposure of the metal precursor film to a sourceof S. As discussed previously, the addition of S complicates thechalcogenization of the metal precursor film and the formation of thechalcogenide material.

Optical absorbers in solar cells and methods of forming opticalabsorbers are disclosed. An optical absorber is part of a thin filmstack in a solar cell. The absorber layer is a CIGS(Se) semiconductor,formed from a precursor film stack which is chalcogenized. The precursorfilm comprises one or more thermodynamically stable layers comprisingCu, Ga, and In, wherein at least one layer comprises a layer rich in oneor more of Cu and Ag, i.e., (Cu+Ag)/(In+Ga)>0.5, wherein the overallaggregate composition of the layers forming the precursor film is0.7<(Ag+Cu)/(In+Ga)<1.0, 0.0<Ag/(Cu+Ag)<0.3, and 0.0<Ga/(In+Ga)<0.5. Insome embodiments, the overall composition of the layers forming theprecursor film is 0.7<(Ag+Cu)/(In+Ga)<1.0, 0.05<Ag/(Cu+Ag)<0.3, and0.0<Ga/(In+Ga)<0.5. The composition of Cu, Ga, and In in each layerexhibits a single phase and the phase remains substantially constant incomposition and laterally uniform in composition when heated above 155°C. The precursor film stack can comprise from one to ten or more layerswith the precursor film stack typically ranging from 400 nm to 800 nm inthickness. Upon chalcogenization, the precursor film forms an opticalabsorber that is typically from 1.0 μm to 2.5 μm in thickness.

In some embodiments, the precursor film comprises a set of two or morelayers, each layer exhibiting a thermodynamically stable single phase.In some embodiments, the precursor film comprises a set of two layers ineach set of layers e.g., a stack of repeating layers, or a stack ofnon-repeating layers. In this embodiment, one layer comprises Cu, In,and Ga, where the atomic ratio of Cu to (In+Ga) in the layer comprisingCu, In, and Ga is greater than 2 and the atomic ratio of Ga to (Ga+In)is greater than 0.5, and the second layer comprises In. In anotherembodiment, one layer comprises Cu, In, and Ga, where the atomic ratioof Cu to (In+Ga) in the layer comprising Cu, In, and Ga is greater than2 and the atomic ratio of Ga to (Ga+In) is greater than 0.5, and thesecond layer comprises an In chalcogenide (e.g., In₂Se₃ or In₂S₃). Thelayers in the precursor film can be present as layer pairs or can bedeposited in any desired sequence to achieve a desired overallcomposition. If desired, one or more additional layers having adifferent composition can be deposited with the layer pairs to achieve adesired overall composition, or to provide grading through the thicknessof the absorber layer.

In some embodiments, the precursor film comprises three layers in eachset of layers, e.g., a stack of repeating layers, or a stack ofnon-repeating layers. In some embodiments, one layer of the stackcomprises Ag and In, containing less than 21% In by weight, the secondlayer comprises Cu and Ga, and the third layer comprises In to provide aprecursor film comprising Cu, Ga and In in the range of0.7<(Ag+Cu)/(In+Ga)<1.0, 0.05<Ag/(Cu+Ag)<0.3, and 0.0<Ga/(In+Ga)<0.5. Insome embodiments, one layer of the stack comprises Ag and In, containingless than 21% In by weight, the second layer comprises Cu and Ga, andthe third layer comprises In₂Se₃ or In₂S₃. The layers in the precursorfilm can be present as layer triplets or can be deposited in any desiredsequence to achieve a desired overall composition. If desired, one ormore additional layers having a different composition can be depositedwith the layer triplets.

In some embodiments, the precursor film comprises three layers in eachset of layers, wherein one layer comprises Cu and Ga, one layercomprises Cu, and one layer comprises In. In some embodiments, theprecursor film comprises three layers in each set of layers, wherein onelayer comprises Cu and Ga, one layer comprises Cu, and one layercomprises In₂Se₃ or In₂S₃. The layers in the precursor film can bepresent as layer triplets or can be deposited in any desired sequence toachieve a desired overall composition. If desired, one or moreadditional layers having a different composition can be deposited withthe layer triplets.

When the precursor film stack comprises sets of two layers, theprecursor film can be present as pairs of each layer, providing anoverall composition for the precursor film which is an average of thecomposition of the two layers. The composition of the precursor film canbe varied by adjusting the number of layers having each separatecomposition provided in the film. Similarly, when the precursor filmcomprises sets of three layers, the precursor film can be present astriplets of each layer, providing an overall composition for theprecursor film which is an average of the composition of the threelayers. The composition of the precursor film can be varied by adjustingthe number of layers having each separate composition provided in thefilm or by adjusting the thickness of particular layers. For example,the embodiments described above can be provided in a range of Incompositions, by providing In layers of variable thickness, or byproviding additional In layers. Similarly, the In composition of theprecursor film can be varied by reducing or increasing the thickness ornumber of layers comprising Cu and Ga.

In some embodiments, the layers in a set of layers do not necessarilyfollow any particular repeating pattern, and can be deposited as neededto provide a desired overall composition or to provide a desiredgradient in composition through the thickness of the precursor film. Inaddition, the precursor film can vary in composition through thethickness by varying the composition, deposition order or thickness ofindividual layers in the set of layers, so long as each layer asdeposited exhibits a single phase. Similarly, if desired, additionallayers can be present so long as each layer as deposited exhibits asingle phase.

In some embodiments, each layer exhibits a single phase in the depositedmetal phase, wherein the metal phase further comprises an additionalphase of alkali salt or other dopant or grading material. For example, asingle phase metal layer (e.g., an In layer, or a single-phase Cu—Gaalloy layer) can be multi-phasic when including an alkali salts (e.g.,sodium) where the alkali salt is part of a salt (e.g., NaF) embedded inthe single-phase metal layer (e.g., In, or single-phase Cu—Ga). Thealkali source can also be part of the single metal phase (e.g., asingle-phase In—Na compound). Where the precursor film compriseschalcogenide layers (e.g., In₂Se₃ or In₂S₃), the chalcogenide can bepresent as a separate phase within a layer that exhibits a single phasein the deposited metal phase.

In some embodiments, the optical absorber can exhibit a graded bandgapformed by a singly graded or doubly graded distribution of Ag relativeto (Cu+Ag) throughout the thickness of the absorber. This can beaccomplished by tailoring the atomic ratio of Ag to (Cu+Ag) throughoutthe thickness of the precursor film comprised of one or more layers. Itshould be understood that depending on the subsequent processingconditions (e.g. selenization) the Ag/(Cu+Ag) depth profile of the finaloptical absorber might deviate from the initial Ag/(Cu+Ag) depth profileof the precursor film.

In some embodiments, the optical absorber can also exhibit a gradedbandgap formed by a singly graded or doubly graded distribution of Ga to(Ga+In) throughout the thickness of the absorber. This can beaccomplished by tailoring the atomic ratio of Ga to (In+Ga) throughoutthe thickness of the precursor film comprised of one or more layers. Inaddition, bandgap grading can be accomplished by tailoring the atomicratio of Ag to (Cu+Ag) throughout the thickness of the precursor filmcomprised of one or more layers. In addition, bandgap grading can beaccomplished by tailoring the alkali content (e.g. sodium) throughoutthe thickness of the precursor film comprised of one or more layers.

It should be understood that depending on the subsequent processingconditions (e.g., selenization), the Ga/(In+Ga) depth profile of thefinal optical absorber might deviate from the initial Ga/(In+Ga) depthprofile of the precursor film. To prepare a solar cell, the precursorfilm is chalcogenized by exposure to heat in the presence of a chalcogensource, either as a film, vapor, or gas, typically the chalcogen sourcebeing Se and S, to form the optical absorber. Furthermore, the opticalabsorber in a solar cell can exhibit a graded bandgap formed by a singlygraded or doubly graded distribution of Se and S through the thicknessof the absorber. It should be understood that the final bandgap gradingthroughout the thickness of the optical absorber (compoundsemiconductor) is the result of the combination of various compositionaldepth profiles, being e.g., Ag/(Cu+Ag), Ga/(In+Ga), or S/(Se+S).

Methods for forming an optical absorber comprise depositing a pluralityof thermodynamically stable layers exhibiting a single phase, whereineach layer comprises one or more of Cu, Ga, and In on a substrate, andheating the layers in the presence of a chalcogen source to effect astoichiometrically relatively complete chalcogenization reaction. Thedepositing can be performed by any convenient method. Some suitablemethods include physical vapor deposition (PVD or sputtering),evaporation, chemical vapor deposition (CVD), plasma enhanced CVD(PECVD), printing, wet coating, or plating. The heating step can beperformed in a batch system or in an in-line system. The chalcogensource can be a film, a vapor, or a gas. A common chalcogen film is Se.A common chalcogen vapor is Se or S. A common chalcogen gas is based onH₂Se or H₂S.

The methods can comprise depositing the layers as one or more sets ofone or more layers. Each set of layers comprises at least two layers,and each layer in each set of layers comprises one or more of Cu, Ga,and In exhibiting a single phase. The roughness of each layer does notchange significantly when heated to 155° C., although both density androughness of the film can change upon heating. The average roughness ofthe precursor film after heating to 155° C. is less than 100 nm, and themin-max roughness is less than 350 nm, as measured in an area of 10μm×10 μm. Each layer has a thickness (roughness) varying by no more than10%. In some embodiments, each layer exhibits an average roughness of nomore than ±˜10 nm as measured in an area of 10 μm×10 μm. In someembodiments, a set of layers comprises two layers. In some embodiments,a set of precursor layers comprises three layers.

The plurality of layers (or sets of layers) form a precursor film fromabout 400 nm to about 800 nm in thickness, and the optical absorber canbe formed by repeating the depositing steps as desired followed by asubsequent heat treatment in the presence of a chalcogen source. In someembodiments, the method comprises depositing from 1 to 10 sets ofprecursor layers to form the precursor film.

The overall composition of the precursor film has a composition definedby atomic ratios of (Cu+Ag)/(In+Ga)<1 and Ga/(In+Ga)<0.5. In someembodiments, the overall composition of the layers forming the precursorfilm is 0.7<(Ag+Cu)/(In+Ga)<1.0, and 0.0<Ga/(In+Ga)<0.5. In someembodiments, the overall composition of the layers forming the precursorfilm is 0.7<(Ag+Cu)/(In+Ga)<1.0, 0.05<Ag/(Cu+Ag)<0.3, and0.0<Ga/(In+Ga)<0.5. The methods can comprise depositing any number oflayers having varying thickness or composition, or varying the order ofdeposition, so long as each layer as deposited exhibits a single phase.Similarly, if desired, the method can comprise depositing additionallayers so long as each layer as deposited exhibits a single phase.

In some embodiments, the methods comprise depositing one or more sets oftwo layers using two PVD targets selected from Cu₂(In_(x)Ga_(1−x)),x=0.25, and In. The one or more sets of two layers are deposited usingtwo PVD targets selected from Cu(In,Ga) and In, wherein the Cu(In,Ga)target has an atomic ratio of Cu to (In+Ga) greater than 2 and an atomicratio of Ga to (Ga+In) greater than 0.5.

In some embodiments, the methods comprise depositing one or more sets ofthree layers using three PVD targets, wherein the first target containsAg and In containing less than 21% In by weight, the second targetcontains Cu and Ga where the Cu and Ga target comprises less than 45% Gaby weight, and the third target contains elemental In, where the layersare deposited in an inert atmosphere. In some embodiments, the one ormore sets of three layers can be deposited using three PVD targets,wherein the first target contains Ag and In containing less than 21% Inby weight, the second target contains Cu and Ga where the Cu and Gatarget comprises less than 45% Ga by weight, and the third targetcontains an In chalcogenide target, for example, In₂(S,Se)₃ with anyratio of S/(S+Se), In₂Se₃ and/or In₂S₃ targets, wherein the layers aredeposited in an inert (e.g., vacuum) atmosphere. In some embodiments,the layers comprising Ag and In, and Cu and Ga are deposited in an inertatmosphere, and the layer comprising an In chalcogenide is deposited inan atmosphere comprising one or more of S and Se.

In some embodiments, the one or more sets of three layers are depositedusing three PVD targets selected from a Cu and Ga alloy target, a Cutarget, and an In chalcogenide target, for example, In₂(S,Se)₃ with anyratio of S/(S+Se), In₂Se₃ and/or In₂S₃ targets, wherein the layers aredeposited in an inert (e.g., vacuum) atmosphere. In some embodiments,the one or more sets of three layers are deposited using three PVDtargets selected from a Cu and Ga alloy target, a Cu target, and an Inchalcogenide target, for example, In₂(S,Se)₃ with any ratio of S/(S+Se),In₂Se₃ and/or In₂S₃ targets, wherein the Cu and Ga alloy layers, and Culayers are deposited in an inert atmosphere, and the In chalcogenidelayer is deposited in an atmosphere comprising one or more of S and Se.In some embodiments, the one or more sets of three layers are depositedusing three PVD targets selected from a Cu and Ga alloy target, a Cutarget, and an elemental In target, wherein the layers comprising Cu andGa and Cu are deposited in an inert atmosphere, and the layer comprisingIn is deposited in an atmosphere comprising one or more of S and Se. Inaddition, when depositing an In chalcogenide target, the depositing canbe performed in a reactive atmosphere (e.g., in the presence of achalcogen source).

In some embodiments, the bandgap is graded through the thickness of theabsorber after heating the precursor film comprising the one or moresets of layers. The methods can further comprise grading the bandgap ofthe absorber layer. The bandgap can be graded by varying the ratio of Agto (Cu+Ag), by varying the ratio of Ga to (Ga+In), or by varying theratio of S to (S+Se) through the thickness of the plurality of layers ofthe precursor film.

In some embodiments, the methods for forming an optical absorbercomprise designating a plurality of site-isolated regions on thesubstrate, depositing a plurality of layers comprising one or more ofCu, Ga, and In on a substrate using PVD to form a precursor film,heating the layers in a chalcogenizing atmosphere to effect achalcogenization reaction, varying process parameters (e.g., PVD andannealing process parameters) among the plurality of site-isolatedregions in a combinatorial manner, and characterizing each precursorfilm or optical absorber formed on the discrete SIRs, for example forphase changes or other parameters as temperature is increased. Theprocess parameters can comprise one or more of wt % In in Ag—In target,wt % Ga in Cu—Ga target, wt % Cu in Cu—In—Ga target, wt % Ga in Cu—In—Gatarget, wt % Cu, Ag, In, and Ga in deposited film or stack of layers,sputtering power, sputtering pressure, sputtering atmosphere composition(e.g., O₂, H₂Se or H₂ in addition to Ar or other noble gas), sputteringtime, substrate temperature, annealing temperature and time, annealingatmosphere composition, annealing pressure, number of sets of precursorlayers, and co-deposition vs. sequential layer deposition. In someembodiments, the characterizing each precursor film or optical absorbercomprises measuring a structure or performance parameter of theprecursor film or optical absorber for each of the plurality ofsite-isolated regions. In some embodiments, the structure or performanceparameter is one or more of crystallinity, grain size (distribution),lattice parameter, crystal orientation (distribution), matrix andminority composition, bandgap, bandgap grading, bulk bandgap, surfacebandgap, efficiency, resistivity, carrier concentration, mobility,minority carrier lifetime, optical absorption coefficient, surfaceroughness, adhesion, thermal expansion coefficient, thickness,photoluminescence properties, surface photovoltage properties, haze,gloss, specular reflection, etc.

Solar cells including the optical absorber can further comprise a backelectrode, a buffer layer, and a top electrode layer, wherein theabsorber layer comprises one or more elements from Cu, and Ag, one ormore from Ga, and In, and one or more of S and Se. Methods for forming asolar cell assembly further comprise depositing a back contactelectrode, an optical absorber, a buffer layer, and a front contactelectrode. An exemplary solar collector can be fabricated on a substratesuch as soda lime glass or stainless steel or aluminum foil. (1) A firstlayer of 200-500 nm of Mo is deposited by PVD for use as a back contactelectrode. (2) The metals for the absorber are then deposited by PVD ina second layer of 400-800 nm (more typically in the range of 500-700 nm)from two or three targets. (3) After the metals are deposited, a batchfurnace or inline furnace is used to heat the layers in an Se and/or Scontaining atmosphere to form an (AgCu)(InGa)(S,Se)₂ absorber layer fromthe second layer. Typically, the finished layer thickness is 1.2-2.4 μm;i.e., the selenization/sulfurization of the metals expands the layer byclose to a factor of three in thickness. (4) 30-70 nm of CdS are thendeposited in a third layer to serve as an n-type buffer layer. (5) Afourth layer comprising a bilayer of ZnO/Al—ZnO is then deposited toserve as a top contact electrode.

In some embodiments, the order of layer deposition is reversed. Thefourth layer can be deposited on the substrate followed by the thirdlayer, and the second layer. The second layer is selenized, and then thefirst layer is deposited.

Various sputtering targets are described above. It is understood thatthe sputtering apparatus and method details can vary. The sputteringprocess can be AC or DC or pulsed DC. Co-sputtering from a plurality oftargets simultaneously can be performed if the apparatus supports such aplurality of sputtering sources. Sequential sputtering from differenttargets can be employed to deposit multiple layers having varyingcompositions. Where metal layers are to be deposited, sputtering cangenerally be performed in an inert atmosphere such as Ar. Masking andaperturing schemes can be used to restrict deposition to a site-isolatedregion of a substrate. In general, any known variation on sputteringmethods can be used with the novel target combinations as long as theavailable process controls can enable deposition of the target layerthicknesses and compositions.

While the above embodiments have been generally described as enablingthe deposition of absorbers having substantially uniform compositionboth laterally and across the thickness, in some embodiments, it isdesired to grade the bandgap across the thickness. Typically, theefficiency of the absorber can be increased by having a bandgap that ishighest at the back electrode (the Mo layer) and that generallydecreases across the thickness of the absorber. In some embodiments, asmall increase near the front electrode can be added. The grading of thebandgap can be achieved by varying any of the relative compositions ofelements from the same column of the periodic table: Ag vs. Cu, In vs.Ga, and/or S vs. Se. Some grading tends to occur due to migration duringannealing (i.e., selenization/sulfurization). Ga, in particular, tendsto migrate toward the back electrode to form a Ga gradient even if nosuch gradient exists before annealing. As such, the “correct” precursorlayer configuration prior to annealing can require experimental testingagainst the actual bandgap grading after annealing.

It can be appreciated that there are numerous process variations to beoptimized for maximizing the efficiency of a laterally uniformcomposition absorber. For graded-band-gap absorbers, there are even morevariations that can be explored and optimized. These optimizations canbe expedited using the High Productivity Combinatorial (HPC) techniquesdiscussed above. While production solar panels are generally made withnominally uniform layer compositions across large device areas, it istime consuming and expensive to make large panels with each experimentalprocess parameter variation. HPC techniques can be used to implement alarge number of process parameter variations in site-isolated regions ona substrate and test each variation for desired absorber or precursorfilm performance characteristics. In the context of the novel PVDmethods for making CIGS absorbers disclosed herein, the processparameters that can be varied in a combinatorial manner include: one ormore of wt % In in Ag—In target, wt % Ga in Cu—Ga target, wt % Cu inCu—In—Ga target, wt % Ga in Cu—In—Ga target, wt % Cu, Ag, In, and Ga indeposited film or stack of layers, sputtering power, sputteringpressure, sputtering atmosphere composition (e.g., O₂, H₂Se or H₂ inaddition to Ar or other noble gas), sputtering time, number of sets oflayers, and co-deposition vs. sequential layer deposition.

It will be understood that the descriptions of one or more embodimentsof the present invention do not limit the various alternative, modifiedand equivalent embodiments which may be included within the spirit andscope of the present invention as defined by the appended claims.Furthermore, in the detailed description above, numerous specificdetails are set forth to provide an understanding of various embodimentsof the present invention. However, one or more embodiments of thepresent invention may be practiced without these specific details. Inother instances, well known methods, procedures, and components have notbeen described in detail so as not to unnecessarily obscure aspects ofthe present embodiments.

What is claimed is:
 1. A method of forming an optical absorber, themethod comprising forming a precursor film on a substrate, wherein theforming comprises depositing a plurality of precursor layers, whereineach precursor layer comprises one or more of Cu, Ga, and In, andwherein each precursor layer exists in a single phase; and forming theoptical absorber by heating the precursor film in the presence of atleast one chalcogen.
 2. The method of claim 1, wherein the chalcogencomprises one or more of S or Se.
 3. The method of claim 1, wherein anoverall composition of the precursor layers has a composition defined byatomic ratios of ((Cu+Ag)/(In+Ga))<1 and (Ga/(In+Ga))<0.5.
 4. The methodof claim 1, wherein an overall composition of the precursor layers has acomposition defined by atomic ratios of 0.7<(Ag+Cu)/(In+Ga)<1.0,0.05<Ag/(Cu+Ag)<0.3, and 0.0<Ga/(In+Ga)<0.5.
 5. The method of claim 1,wherein the plurality of precursor layers comprises one or more sets oftwo or three layers.
 6. The method of claim 1, wherein the depositing isperformed using physical vapor deposition (PVD).
 7. The method of claim6, wherein one or more sets of three layers are deposited using threePVD targets, wherein one PVD target comprises Ag and In, one PVD targetcomprises Cu and Ga, and one PVD target comprises In, wherein the PVDtarget comprising Ag and In comprises one of the followingcompositions: 1) less than 21% In by weight, 2) 26-35 wt % In, or 3)AgIn₂.
 8. The method of claim 7, wherein the deposition using the PVDtarget comprising In is performed in the presence of a chalcogen, or thedeposition using the PVD target comprising In is performed in an inertatmosphere and the target comprises In₂Se₃ or In₂S₃.
 9. The method ofclaim 7, wherein the Cu and Ga target comprises less than 45% Ga byweight.
 10. The method of claim 7, further comprising grading a bandgapof the absorber layer by varying the atomic ratio of Ag to (Cu+Ag)through the thickness of the precursor film.
 11. The method of claim 6,wherein one or more sets of two layers are deposited using two PVDtargets, wherein one PVD target comprises Cu₂(In_(x)Ga_(1−x)), x=0.25,and one PVD target comprises In, wherein the deposition using the PVDtarget comprising In is performed in the presence of a chalcogen, or thedeposition using the PVD target comprising In is performed in an inertatmosphere and the target comprises In₂Se₃ or In₂S₃.
 12. The method ofclaim 6, wherein one or more sets of two precursor layers are depositedby PVD using two PVD targets, wherein one PVD target comprises Cu(In,Ga)having an atomic ratio of Cu to (In+Ga) greater than 2 and an atomicratio of Ga to (Ga+In) greater than 0.5, and wherein one PVD targetcomprises In.
 13. The method of claim 12, wherein the deposition usingthe PVD target comprising In is performed in the presence of achalcogen, or the deposition using the PVD target comprising In isperformed in an inert atmosphere and the target comprises In₂Se₃ orIn₂S₃.
 14. The method of claim 6, wherein one or more sets of threeprecursor layers are deposited using three PVD targets, wherein thethree PVD targets are one of: 1) wherein one PVD target consistsessentially of Cu and Ga, one PVD target comprises Cu, and one PVDtarget comprises one or more of In₂Se₃ and In₂S₃, wherein the layers aredeposited in an inert atmosphere, or 2) wherein one PVD target consistsessentially of Cu and Ga, one PVD target comprises Cu, and one PVDtarget comprises In, wherein the layers comprising Cu and Ga and Cu aredeposited in an inert atmosphere, and the layer comprising In isdeposited in an atmosphere comprising one or more of S and Se.
 15. Themethod of claim 1, further comprising grading a bandgap of the absorberlayer by varying a ratio of Ga to (Ga+In) through a thickness of theplurality of precursor layers.
 16. The method of claim 1, furthercomprising grading a bandgap of the absorber layer by varying a ratio ofS to (S+Se) through a thickness of the plurality of layers after theheating.
 17. A method of forming an optical absorber comprisingdesignating a plurality of site-isolated regions (SIRs) on a substrate,forming a precursor film on each of the SIRs, wherein the formingcomprises depositing a plurality of precursor layers, wherein eachprecursor layer comprises one or more of Cu, Ga, and In, and whereineach precursor layer exists in a single phase; varying processparameters during the forming among the plurality of SIRs in acombinatorial manner; forming the optical absorber by heating theprecursor films in the presence of at least one chalcogen; andcharacterizing each optical absorber formed on each SIR.
 18. The methodof claim 17, wherein the process parameters comprise one or more of wt %In in Ag—In target, wt % Ga in Cu—Ga target, wt % Cu in Cu—In—Ga target,wt % Ga in Cu—In—Ga target, wt % Cu, Ag, In, and Ga in deposited film orstack of layers, sputtering power, sputtering pressure, sputteringatmosphere composition, sputtering time, number of sets of layers, andco-deposition vs. sequential layer deposition.
 19. The method of claim17, wherein characterizing each optical absorber comprises measuring astructure or performance parameter of the optical absorber.
 20. Themethod of claim 19, wherein the structure or performance parameter isone or more of crystallinity, grain size (distribution), latticeparameter, crystal orientation (distribution), matrix and minoritycomposition, bandgap, bandgap grading, bulk bandgap, surface bandgap,efficiency, resistivity, carrier concentration, mobility, minoritycarrier lifetime, optical absorption coefficient, surface roughness,adhesion, thermal expansion coefficient, thickness, photoluminescenceproperties, surface photovoltage properties, haze, gloss, specularreflection.